Re-Wire Miles Platting 70's guitar amp back to original specs

[raketenmann]

Dear community

A couple of years ago I acquired an old Miles Platting 100W guitar/bass head. What I didn't know is that the amp was heavily modified. So the amp needs a service which includes changing of the filter caps and probably even more since the amp is noisy and starts oscillating at some volume.

However, the amp has been modified so wildly and without safety standards so that I can't find a tech to lay hands on at a reasonable price. So I am in the situation to either dump the amp completely or just put a lot of effort into it and just rebuild it back to stock, which is what I tend to do at the moment.

However, I am not an electrician neither an engineer. I know some basic stuffs about amps, now where the dangers of filter caps are and I already built s small tube amp and some guitar pedals before. My plan would be to gather all the infromations I need to rebuild the stock circuit, do all the soldering, show it to a repairsmann who checks the power stage and then adjusts the Bias.

However, this means that there are a lot of open questions and I would really be thankful if there are some people that would be willing to share their thoughts and time to give me a little guidance through this.

So first things first: I am currently creating a bill of materials. Some of the original resistors and caps are probably still inside the amp. My question is: should I just dump all the old electonics and replace them or should I try to recycle some of the old parts? Based on what knowledge whould you make your decision? Also, If I am to replace resistors and caps: What are your suggestions about the materials to use?

I am lucky that I know an older man who has a tube tester and the old original tubes (Mullard EL34 and Brimar 12AX7) are still in working order with a lot of life left in them. The power transformer has been replaced, but the OT is still the original one.

Another question: this amp is soldered completely freely without a board. This makes it a lot more difficult for me as a Rookie to get it all together. So my question is: I know it would be possible to build the whole preamp section onto a turret board. But obviously I am not the guy to do so. Are there people that are able to do draw a board on the computer without too much effort and would be willing to do so? That would be so much help!

And while taking about preamp circuit: The amp offers two channels that are completely identical. Probably it would be fun to modify one of the two a little different.... any ideas?

Thanks a lot for all your help already!

PS: in the attachments you'll find all the shematics and a picture of the original amp inside and one of my modified example...

 

[Disco Volante]

It looks like you've got most of the information that you need.
The picture of the original unit together with the schematic and a good amount of long evenings patiently reverting, replacing, making list after list as you go, would get you there eventually.
Don't expect to be able to make a complete bom before you start, there's always something that comes up along the way.
Replace all electrolytic caps and those pesky tantalums too (if there are any).
Resistors are mostly hard-wearing. Replace any that look burned/brown. Later if the amp makes trouble, you can start measuring them.
That's how I go along. Other people probably have other ways.

Good luck and happy tinkering

Just saw this post a month late, not sure why... Hope you're already working on that amp;-)

 

[Disco Volante]

Taking a closer look at hte pictures, looks like someone really hacked this beast. It's really quite a simple amp from the start. Without tracing the new schematic, it's hard to say if any changes are worth keeping. But that wasn't the plan anyway;-)

 

[NOON]

Watch some Mr Carlsons Lab and Brad's Guitar Garage on yourube for how to do this kind of work to a high standard.

 

[capacitorless]

This looks like the proverbial medicine ball of amp repair, haha! Not meaning this negatively in any way. It's kind of the opposite of rebuilding a Champ in terms of time and skill level, so you'll be learning a lot. Obviously the usual warning about lethal voltages and filter caps apply.

If it was on my bench, I would rebuild the power supply and output section first, paying special attention to the bias supply and the two 10k pots that adjust bias in the power section. If those are funky, you'll run the risk of overheating/melting/frying one or more EL34s and/or the power transformer. It's not super hard, just attention to detail here is a good thing I would typically replace both those pots with higher grade, multi-turn ones. It's cool that they already have two pots in there, rather than a single pot that requires all 4 output tubes to be more closely matched.

You don't need them to get going, but the one thing I would think about adding would be soldering in 1 ohm resistors in each of the EL34's cathodes (between the cathode and ground). You can use them later to verify the current each tube is drawing - makes it easier to balance bias, weed out a cranky tube etc.

Once you've done that, you can get some gratification by plugging a signal source into the phase inverter input, to see what it sounds like without preamp overdrive/eq. It probably won't be super loud, but you can see if it seems like it's worth it, before proceeding to the preamp.

Something useful would be a light bulb based shorts detection box. Variac would be great, but I've gotten by without, and if you're replacing all of the power supply caps you should be ok. And definitely make sure the line fuse is intact; sometimes people bypass it by sticking something conductive in there. Have a few 3A fuses of the proper speed (fast vs slow) handy.

 

[raketenmann]

Hey there!

So nice that after a while some people are getting involved in my project

But as time went by I changed my mind and my plan is now just to fix the issues the amp has (feedbacks at higher volumes). So I already replace all electrolytic capacitors and some Capacitors that were leaking DC voltage. Also I will be replacing the screen grid resistors. After that I will be done for the moment and check if the amp is back playing normal. I think built the amp backwards is just to much work and a too hard for me to deal. As I said: I am not skilled in electronic repairs. I just know the basics, including how to discharge caps and work safely. But at the momen, I do have some specific question some of you may answer...

1) I just did bias adjustment on a fender amp. So here, I am actually not really aware how to do it. You say there are two pots to do so. Are these the ones marked on the photo? I do have a bias probe to put into the tube socket. So should I put it one of the two left sockets, adjust bias, turn the amp off, put it into one of the two right sockets and adjust the second pot or how is this working? Also, what mA should be a good starting point for this amp?

2) I did only direct replacements (caps & resisitors), very patiently and with high accuracy. So I am 100% sure I did the replacements without any faults in the circuit. I don't have a variac or such. Is there anything I can do except for just turn it on and hope that it will not go up in flames?

3) There are two other adjustment pots I actually don't know what they are for. One is directly on the first EL34, parallel to the heater. The other one is over the 12AX7 phase inverter stage (see photos). What do they do and should I make adjustmens to them?

I know, it is pretty wild, but as I said: DIY is the only way for me, the techs refused to do it for a reasonable price. If I can't do it, I will need to sell the amp as it is for probably nothing... So I am really looking forward to somehow get it done with a little help from this great community here.

 

[Disco Volante]

Like @capacitorless hinted at, the old lightbulb-in-series with amp trisck will save your PT in case of a short.
Or just fire it up and watch/listen, sometimes a temperature-laser-probe can help detect hot spots (saves you poking in the HT;-)

No idea what a pot parallel with the heater would do, maybe an attempt at a humbucker?
If you're able to trace the relevant area, a sketch diagram of how the extra pots are connected would help.

The 10k Rs in the cathodes are your friends. Measure that they all are pretty close on spec, and then set your bias by simply measuring the voltage drop over each. Ohm's law will tell you how much your idle current is.
No need to unplug anything. Ideally, each tube should be biased separately, but if that's not possible, a good balance either side of the OPT will go a long way.

Happy tinkering!

 

[capacitorless]

Yes, pretty certain the two circled ones in the first photo are the bias pots, which are shown in the schematic as 10k. You could verify the value with a DMM of course. If keeping, I would put some pot-friendly electronics cleaner on them, and rotate them fully a few times. That might save a couple of power tubes and/or the output transformer someday.

Edit: the more negative voltage bias that a tube sees, the less power it transfers. If a bias pot opens up, however, it means there's no bias on the associated tubes - eg a 0v bias voltage, if you will. This leads to the tubes heating up way more than they should be, and drawing excessive current through that side of the output transformer. Bias pots get dirty and noisy too, especially the non-enclosed type. This typically means there's a good chance of a vibration causing the pot to open up (and provide no output voltage) while being used, if not clean.

I would also suggest ampgarage.com, if you aren't on there already. Lots of friendly people experienced with amp modding/fixing/building hang out there

 

[raketenmann]

The 10k Rs in the cathodes are your friends. Measure that they all are pretty close on spec, and then set your bias by simply measuring the voltage drop over each. Ohm's law will tell you how much your idle current is.
No need to unplug anything. Ideally, each tube should be biased separately, but if that's not possible, a good balance either side of the OPT will go a long way.

OK - but as I said I just bought the tube socket to do the bias adjustment, I would like to do it that way, I suppose it it also safer...?
I just don't actually know how to do it: Do I need to measure the HT or can I rely on the numbers on the schematic? I want to use this tool here:

https://robrobinette.com/Tube_Bias_Calculator.htm

But I just don't know where to get all the needed specs...

Yes, pretty certain the two circled ones in the first photo are the bias pots, which are shown in the schematic as 10k. You could verify the value with a DMM of course. If keeping, I would put some pot-friendly electronics cleaner on them, and rotate them fully a few times. That might save a couple of power tubes and/or the output transformer someday.

Thanks for the tipp. I will some gentle cleaning of the pots!

No idea what a pot parallel with the heater would do, maybe an attempt at a humbucker?
If you're able to trace the relevant area, a sketch diagram of how the extra pots are connected would help.

I will try to check if I can figure out where these pots are connected. As already said, one is prett straight forward over the first EL34, the other one sort of between the filter caps and the PI. I try to figure it out, but I do need some time. Looking at the pictures of an original amp I see that both of these pots are not original....

 

[GussyLoveridge]

Measure your plate voltage from pin 3 to ground. Do be very careful when you are measuring inside a live amp. I am assuming you wouldn't be working on this amp without a serious amount of caution and respect for how dangerous it is.

I'm not sure what your bias measuring tube socket does exactly, I have one that hooks up to my DMM to measure plate current. If yours does the same you'll measure your plate current.

P=I*V -----Make sure you're using V and A not mA.

Multiply your plate current by your plate voltage and you'll get your plate dissipation in watts.

An EL34 has a max plate dissipation of 25 W.

Take your plate dissipation (W) and divide by 25 (W) - you'll be left with the percent dissipation. I try and keep it within 50% to 75% or so. And I'll adjust and play until I'm happy with the sound, or the end user is happy with the sound. Sometime my version of happy with the sound puts me closer to 80-85%. I believe the higher % dissipation, the shorter the tube life might be.

All of this is probably slightly simplified version of what is actually going on (as your screen current may also be included in your measurement depending on how you take it) but it has worked well enough for my use over the past 15 years.

There is so much information available on all of this stuff online. Be careful, apparently not everything you read on the internet is true.

 

[raketenmann]

Measure your plate voltage from pin 3 to ground. Do be very careful when you are measuring inside a live amp. I am assuming you wouldn't be working on this amp without a serious amount of caution and respect for how dangerous it is.

I'm not sure what your bias measuring tube socket does exactly, I have one that hooks up to my DMM to measure plate current. If yours does the same you'll measure your plate current.

P=I*V -----Make sure you're using V and A not mA.

Multiply your plate current by your plate voltage and you'll get your plate dissipation in watts.

An EL34 has a max plate dissipation of 25 W.

Take your plate dissipation (W) and divide by 25 (W) - you'll be left with the percent dissipation. I try and keep it within 50% to 75% or so. And I'll adjust and play until I'm happy with the sound, or the end user is happy with the sound. Sometime my version of happy with the sound puts me closer to 80-85%. I believe the higher % dissipation, the shorter the tube life might be.

All of this is probably slightly simplified version of what is actually going on (as your screen current may also be included in your measurement depending on how you take it) but it has worked well enough for my use over the past 15 years.

There is so much information available on all of this stuff online. Be careful, apparently not everything you read on the internet is true.

Thanks for your help. My bias adapter measures the indirect "Ruhestrom" over a measuring resistor. I don't know the translation from "Ruhestrom" but I suppose it is the BIAS current. It is this product:

https://www.tubetown.net/ttstore/de/bias-messadapter-oktal-fertigadapter.html

I believe I will figure it out, only the thing with the plate dissipation I don't understand. But I will do some reading about it.

So today I checked how these two odd pots are wired:

Pot PI
Pin 1 pot goes to Pin 6 PI over 82K resistor
Pin 2 goes to the input stage (not 100% sure
Pin 3 goes to Pin 1 PI over 100k resistor

Pot EL34
Pin 1 goes to Pin 2 filament
Pin 2 goes to Pin 1 and Pin 8 (suppressor grid & cathode) and from there to ground
Pin 3 goes to Pin 7 filament

So if anyone could have a clue why someone put in these pots I would be very happy to know!

 

[GussyLoveridge]

I suspect, as someone above had mentioned, that the pot attached to the EL34 is a humdinger to try and reduce noise from the filament supply. Both the sup grid and cathode are tied to ground. This is somewhat of an adjustable, artificial center tap for the filament circuit.

The PI pot I think is an attempt to balance the phase inverter, though I could be way off.

As a note on my philosophy when it comes to working on amp that has been highly modified: I love to draw out what was modified and try and learn why. I almost universally try for amps that I'm keeping to bring them back to their original stock circuit. I prefer to hear them as the designer wished them to be heard and then make decisions as to what may make them function more to my specific tastes. Most of the time, the stock circuit sounds fantastic to me and I leave it that way. Occasionally, the addition of something like the humdinger mod that was added to your amp can clean up a problem, more often I would say that a nice tidy re-dressing of the filament wires does the trick.

If you are considering learning more about tube/valve guitar amplifiers - consider purchasing and studying Merlin Blencowe's books and while you're at it, check out his website. He is a member here and a wealth of knowledge. I have been working through his books for years and understand a little more each year.

https://www.valvewizard.co.uk/

I'm very excited for you! Good luck!

 

[raketenmann]

I suspect, as someone above had mentioned, that the pot attached to the EL34 is a humdinger to try and reduce noise from the filament supply. Both the sup grid and cathode are tied to ground. This is somewhat of an adjustable, artificial center tap for the filament circuit.

The PI pot I think is an attempt to balance the phase inverter, though I could be way off.

As a note on my philosophy when it comes to working on amp that has been highly modified: I love to draw out what was modified and try and learn why. I almost universally try for amps that I'm keeping to bring them back to their original stock circuit. I prefer to hear them as the designer wished them to be heard and then make decisions as to what may make them function more to my specific tastes. Most of the time, the stock circuit sounds fantastic to me and I leave it that way. Occasionally, the addition of something like the humdinger mod that was added to your amp can clean up a problem, more often I would say that a nice tidy re-dressing of the filament wires does the trick.

If you are considering learning more about tube/valve guitar amplifiers - consider purchasing and studying Merlin Blencowe's books and while you're at it, check out his website. He is a member here and a wealth of knowledge. I have been working through his books for years and understand a little more each year.

https://www.valvewizard.co.uk/

I'm very excited for you! Good luck!

Hey there

OK, thank you for your help so far. I actually did build a lightbulb limiter and was able to put the amp to work. It works and there is sound. However, the lightbulb light a bit more than when with two of my other amps (but they are only 40 watters). However, when I turn the master to max and also rise the gain into its limits, the amp starts feedbacking and then the lightbulb starts to glow even more.

So I measure a plate voltage of 516V. I guess this could be to high. Also, with my tube socket measurement tool with a 1,1Ohm resistor I measure 55mV. My calculations say, that a reading of 32mV should be the 60% bias goal. But I can't turn the BIAS pot down, because it is already at its limit.

So I guess there's something wrong, but I suspect it is the same it was before my repair attempt. As I said, the feedbacking issue was the thing that got me into the repair....

any ideas?

 

[GussyLoveridge]

Maybe use a voltage divider after the 100 uF / 100 V cap in the bias circuit to lower your bias voltage to a more reasonable level?

 

[raketenmann]

Maybe use a voltage divider after the 100 uF / 100 V cap in the bias circuit to lower your bias voltage to a more reasonable level?

I don't have experience with voltage dividiers. But first things first:

The bias supply has been modified in the past. Probably because the PT is non original. The PT has a 60V winding for the bias supply, originally there was a 40V supply. So I suppose the bias supply was modified to deal with the new PT. So the cap is a 1000µF instead of 100µF, the resistor is a 2,2k instead of a 3,9k. Over the bias cap I measure a voltage of 45VDC.

So I actually don't understand much about tube amps, and also do not of bias supply designs. So probably someone can give me some advice. My questions are:

- I don't understand how to read the shematic. Where is the bias supply actually going? It seems like it is not really drawn out in the shematic. I also don't know what resistors in the shematic are the two bias pots.

- If the bias pots are not able to get the correct bias - what could be the culprit? Is it only a not correct bias supply or does the plate voltage have to do something with it. As I said in a post before: I have a high plate current around 516V and I am still not sure if this is correct.

- So if plate voltage has nothing to do with it: could I just change resistors to get the bias down? There is as I said a 2,2k resistor. Should I change this? What value? There are also two resistors in front of both bias pots. I could probably change these as well, but as I said: i can't determine what these are in the circuit because I don't see where the bias supply is connected to....

I hope you could help me out! THX

 

[GussyLoveridge]

https://en.wikipedia.org/wiki/Voltage_divider

Check out the section on resistive dividers. They're very powerful!

Your bias pots are essentially variable resistive dividers. That's what gives you the ability to adjust the bias voltage. Turn in one direction will shunt more voltage to ground, turned the other presents a high voltage to the control grids.

Here is some great information on bias supplies.

https://www.valvewizard.co.uk/bias.html

However, when I turn the master to max and also rise the gain into its limits, the amp starts feedbacking and then the lightbulb starts to glow even more.

Others folks here might have other opinions on this, but this sounds like a parasitic oscillation to me within the audio band.

https://en.wikipedia.org/wiki/Parasitic_oscillation

Would you want to be playing the amp with the master at max and gain at maximum as well? (Maybe you do! No judgement here. That sounds fun!) But if not maybe that isn't a problem if it's stable for normal use.

 

[GussyLoveridge]

I'm sorry - I'm realizing I'm probably not doing a great job of helping out here.

I've circled the 10K bias pots in yellow. Blue path presents the voltage to the tube control grids. Black squiggly lines show the shunt path to ground/earth. The bias supply down the bottom comes from a specific tap on the PT, is dropped by the 3.9 K resistor, rectified by the diode and filtered by the 100 uF/100 V cap (which I think, both the R and C values have been changed here) - then continues on to where I've written FROM SUPPLY. Those points are attached - just not on the schematic to it doesn't clutter the diagram up.

I think you could increase the 3.9K resistor before the diode to drop more voltage. Make sure it has ample power handling capabilities (I don't think there is much current here anyway). I think this is where I would start. Maybe messing with what is between the control grid and ground changes the gird leak resistance and I'm not sure that we want to do that. I think it would change its interaction with the previous stage.

I hope this is helpful. I am not an expert at all - I am very much an amateur enthusiast. There are likely other ways to approach this, perhaps better ways. I'm just kind of going through the motions of how I'd figure this out from my limited knowledge.

I really hope this helps. You will figure it out and it will be wonderful. It's very exciting stuff!

Please continue to be careful around these high voltage circuits!!

 

[Disco Volante]

Seems like your HT is within range, valve voltages aren't usually very exact. I'd concur with Gussy, see if you can lower the bias a bit. There's very little current flowing, so a larger resistor should take care of it.
If your plates aren't glowing, the amp isn't going to melt down;-)
The oscillation issue could very well be due to some of all the hacking this amp has been subjected to. Valve circuits are high-impedance, which means things like routing of leads and component placement has effect on stray capacitances.

Just another amateur enthusiast chiming in;-)

Good luck

 

[raketenmann]

View attachment 140327


I'm sorry - I'm realizing I'm probably not doing a great job of helping out here.

I've circled the 10K bias pots in yellow. Blue path presents the voltage to the tube control grids. Black squiggly lines show the shunt path to ground/earth. The bias supply down the bottom comes from a specific tap on the PT, is dropped by the 3.9 K resistor, rectified by the diode and filtered by the 100 uF/100 V cap (which I think, both the R and C values have been changed here) - then continues on to where I've written FROM SUPPLY. Those points are attached - just not on the schematic to it doesn't clutter the diagram up.

I think you could increase the 3.9K resistor before the diode to drop more voltage. Make sure it has ample power handling capabilities (I don't think there is much current here anyway). I think this is where I would start. Maybe messing with what is between the control grid and ground changes the gird leak resistance and I'm not sure that we want to do that. I think it would change its interaction with the previous stage.

I hope this is helpful. I am not an expert at all - I am very much an amateur enthusiast. There are likely other ways to approach this, perhaps better ways. I'm just kind of going through the motions of how I'd figure this out from my limited knowledge.

I really hope this helps. You will figure it out and it will be wonderful. It's very exciting stuff!

Please continue to be careful around these high voltage circuits!!

Thx a lot. Actually your explanaitions help me A LOT! Suddenly I really can see clearly!

OK, another person in another forum said, that the purpose of the resistor in the bias supply is to limit the amps for the cap and shouldn't be messed around. However I did measure the DC over the bias supply cap and it is 45V. Is the bias supply all about voltage? Nothing to do with amps? because I would suspect that that 45VDC is pretty close to the 40V specified in the shematic, so I still try to figure out why my pots are not able toilet cover the needed range. So I actually try to understand two things: 1) HOW and WHAT change of resistors to perform and 2) WHY my current wiring is providing a too high voltage...