Question on a Tube Guitar Amp

[zayance]

Hi Guys, i had a question regarding a Fender Twin reverb Silverface 135W RMS (1979) that i have on my bench.
A friend of me wanted me to check the amp and see if there are any failed components etc..
That obviously leaded to two Filtering caps that had to be replaced, and so i decided to replace them all, and i had to replace some other small things as well.
At first i had loaded some old schematic (aa769 and aa270) missing the good one huh, and it was not looking and measuring near what i had, and at first i tought it was a modded one, below is the correct one, with a HT at 500VDC  , looks a little high for the life of tubes, and i guess he'll need to always make sure to grab some tough ones, he is actually using matched pairs of JJ 6L6GC, and they seem to be up to the challenge (seems at limits tough), while i think a lower HT would be nice to have here, using a Big Resistor for example, or some other method? What do you think?
The main difference along with some others, from the old schematic, is the rectifier, new one is a full wave bridge, while the old is a Full Wave rectifier, i don't have the snubbers on the bridge rectifier tough...It's a point to point build btw.
It's a safe amp to work with, cause it seems there is already a bleeder installed a 30K 20W.

http://ampwares.com/schematics/twin_reverb_sf_135.pdf

Ok My question is regarding the biasing of the amp, i wanted to check that as well, and when i do the simple method of the 1 Ohm on Cathode to GND, i read 5mV wich makes 5mA, thats' a little odd and looks very low for what that amp can deliver. (EDIT: Tubes IN and Speaker IN)
On this amp, and as you can see on the schemo, there is a "hum Balance" control, so current balancing of the Heaters (if i got it correct?), and "Output Tube Matching" control, wich will perform about the same i guess but for the pair of tubes, but that doesn't look like a real bias adjustment...
Id like to know how i could mod it for beeing able to really adjust the bias, with a pot would be easier as always, i understand that a simple method would only be an adjustment for all 4 of the power tubes, and not by pair or by Tubes, but that's better than nothing here...
But maybe there is something else going wrong for having 5mA on each tubes.

Anyway there are couple of things on the web concerning Bias mod on Fender amps, but i'd like to have your toughts here, i could have posted in an Guita amp forum, but i'm use to here, and there are Masterminds and courtoisie, wich is important to me as well...


Thanks a lot for any feedback, i'm not a pro in the Tube Amp domain, so i'd like some toughts...

 

[CJ]

that is probably a PC board amp so changing the circuit is not as easy as the old turret board models,

i would leave the stock circuit and use resistors in parallel to trim the bias to your desired minus DC voltage, parallel 2.2 k to make the bias more neg, parallel 3,3 k to make it more pos,

are you sure you have 1 ohm and not 0.1 ohm resistor on cathodes?

is DMM set to DC ?

to check current place your amp meter across the output transformer pri from red to brn for one set of tubes and red to blue for the other set, hook it up before you turn power for safety, this is a standard method used by experienced techs, not a hairbrain method  that i just thought of, Ken Fisher of Trainwreck, Gerald Weber of Kendrick, they all use this method for quick troubleshooting, probably about 80 years experience combined by those 2 guys so don;t worry about it,

you will be reading plate current in ma, the current would rather go thru the meter than the DCR of the OPT as electricity will always take the path of least resistance.

 

[zayance]

Thanks for the reply CJ

that is probably a PC board amp so changing the circuit is not as easy as the old turret board models,

No it's P2P

are you sure you have 1 ohm and not 0.1 ohm resistor on cathodes?

yes 1R :/

is DMM set to DC ?

Yes of course

to check current place your amp meter across the output transformer pri from red to brn for one set of tubes and red to blue for the other set, hook it up before you turn power for safety, this is a standard method used by experienced techs, not a hairbrain method  that i just thought of, Ken Fisher of Trainwreck, Gerald Weber of Kendrick, they all use this method for quick troubleshooting, probably about 80 years experience combined by those 2 guys so don;t worry about it,

hmmm, thanks, ok i'll try this and report back.


Thanks.

 

[zayance]

8mA from red to Blue
10mA from red to Brown

Speaker In and Tubes IN, just for info.

 

[ruffrecords]

The output tube bias seems to around -50V which is close to cut off this this is definitely a class B amp. The data sheet majors and class AB1 with about 100mA idle current and -25V bias. So the 500V, which is at the max rating, will only be there at no signal idle. As soon as there is signal I expect the HT will droop a lot.

Definitely dodgy but typical guitar amp design.

Cheers

Ian

 

[Dtube]

Hi. Take a look at the Twin AB763 schematic and then use it to convert that 135's silly bias-balance to a true bias adjust. You may then need to play around with the dropping resistor there to get the bias into a workable range. UL 135 Twins run high Ep. 490-500v is not uncommon.
-Darren

 

[CJ]

i would second Dtube on this as long as it is P-P,
put a bias pot in place of the balance pot,

those tubes are running very cold, the tube type  might have something to do with this,

you will have to wire in the Vibrato to the bias like the AB763 also.

a little mis-match is not a bad thing, in fact some people like it a bit better,
i bet the tube current becomes more matched when they are driven harder,
so i would not worry about losing the match pot, you can also swap tubes around to get better balance,

 

[zayance]

hmmm allright, that's what i had bumped to when doing some search, but needed a second tought.
i'll compare both schemos, but seem to be easy enough.
Would need to change a couple of resistors, an rewire things...

Just to be sure, here would be the mods if i got it right:

- There is a 80uF 75V cap beside the 50uF 50V (AB763), so i guess i'll have to desolder the 80uF and change its position to one end of the Balance pot. EDIT: i think i got it wrong that could be left there i guess, the correct one should be the 70uF 100V) that just needs to be wired on one end of the balance pot, done deal.
- Desolder the 47K and 68K resistors going to grid1 of te Power tubes, and install 2x 220K across both pairs, and take middle point as the wiper on the balance pot.
- Add a 27K resistor on the other end of the Balance pot to Gnd (or case of the pot)
But actually, and i think i've read that somewhere that i could substitute the 27K
to a say 50K or 100K pot, so to make the Bias adjustment easier then swapping resistors for the idle? Is that correct?

Concerning the Secondary wiring of the Power transformer to the modded circuit, i'm not sure, maybe you could clarify a bit, that would be nice, but i guess i can just use it as is, meaning the 2.2K resistor IN, and just mod from there....

you will have to wire in the Vibrato to the bias like the AB763 also.

Yes i saw, thanks, needs to be on one end of the Balance pot beside beeing on the "center tap"? of it...

But isn't that odd to have such a small amount of mA for Bias? I mean the guys was using it as is and
the Amp works tough, but because of that you have to push the volume pretty high to get some beef
and the beef is sounding awful...
Anyway i'll mod to that Blackface Bias control and report back.

Thanks a lot guys

 

[zayance]

those tubes are running very cold, the tube type  might have something to do with this,

BTW, datasheet of the tubes

http://www.jj-electronic.com/pdf/6L6%20GC.pdf

 

[CJ]

ok so i get 20 ma at 500 and -50 bias for the JJ's off the load line and 40 ma off the RCA loadline,
(click on Franks at the bottom of ths post)

so if you want to match the idle current of the JJ's to RCA, drop the bias down to -45 VDC and i bet the tone will improve.

JJ makes some great tubes but they do run a bit different, at least the 6V6 and the 6L6 stuff that we run over here,

 

[zayance]

Ok Thanks for the values.

But one thing bothers me here is that you have to crank the gain and master volume nearly at max to get some decent volume, and that's weird for a 135W, i mean it should be crazy loud at that point.
I hope the Output transformer is ok, the sound comes out, but i'm wondering now...
What would be the best way to test it out? Maybe i'm wrong but i'd like to be sure....

Also the Master volume is push pull switch, what does it make exactlly?

 

[zebra50]

Something sounds wrong there, Tony. All the fender amps I have come across have been insanely loud, even the 50 watt devilles are a menace in a small room.

 

[Dtube]

The P-P master engages a switch that swipes some gain off the reverb driver to make a crude -and generally crummy-sounding- boost. If your bias current is indeed only 5ma, that is why you have low-gain. That output section is almost biased into shut-off.
-D

 

[CJ]

put an AC meter on a 6L6 grid and see what the drive voltage is,
for a -50 volt bias, full output will require about half of this if you figure in the RMS vs Peak voltage thing,

also, some people like the ultra linear transformer, some do not,
it is easy to change back to a blackface circuit, just clip and safe off the UL leads, and wire the screens like the AB763.

 

[zayance]

@Dtube: allright i get it, yes from graph it seems super low, will see with the bias readjusted...

@CJ: i'll do that tomorrow..., so the blue wire goes for one pair of screens and brown for the other, yes easy mod.
For now i'll concentrate on that low bias, and see how all goes, will try that blackface OT mod afterwards...

Thanks a lot guys, much appreciated.


T.

 

[Dtube]

I've found that doing the BF mods on the phase inverter and bias supply, but leaving the UL screens intact does wonders for these amps. I've tried bypassing the UL taps and adding 470-ohm resistors on the tube sockets really wasn't worth the additional effort. I'm not sure if it would fit the mounting holes, but a traditional BF Twin PT would probably do as much for one of these amps as anything in the tweaking department. I blame the tone more on the high Ep than the UL transformer. But, YMMV as this is JMHO.
-Darren

 

[CJ]

blu/striped and brn/striped are the screens, blu and brn or the plates,

check your preamp tube voltage, as Dtube suggests, HV = Harsh, brittle hi end,

you might like the sound of lower B+ feeding the input tubes,

yes, removing the UL taps is a very subtle change so maybe not worth it,

 

[zayance]

blu/striped and brn/striped are the screens, blu and brn or the plates,

Yes my bad.

I'll do all test and report back....

 

[PRR]

> 5mA, thats' a little odd and looks very low for what that amp can deliver.

Idle bias has almost no effect on Maximum Power.

You want the bias low for not-HOT idle, but high enough that it does not sound raspy (crossover distortion) around the 1 Watt level. (In this case, you could simply PLAY LOUD and never care about stray THD near 1W.)

Fender has traditionally biased pretty cool. It may be "part of the sound". At extreme, it widens the dynamic between background and slam.

> using a Big Resistor

No good. Here's why. Say you like 450V. And idle is 5mA each, 20mA total. 50V/20mA= 1,200 ohms. That will hit 450V at 20mA. Now PLAY HARD. These tubes will go toward 200mA per pair, 400mA. Compute: 2,500 ohms times 0.4 Amps is 6 thousand volts..... uh, that can't happen. Actually it will suck down to zero with 200mA of tube current. Tubes can't suck big current at zero volts. I think at full output you will be near 150V B+ and eleven Watts output.

Yes, you can bias hotter. Probably should. Taking 500V and "35W" and the mysterious 70% rules, 50mA per tube is the max good bias. (Once again: be sure your 5mA isn't already 50mA!! I make that mistake a lot.)

> 500VDC, looks a little high for the life of tubes

It's totally fine. Original 6L6 was 360V-400V because of the base material, not the guts. Improved base material is now standard. 6L6GC is a very different beast, derived from a TV H-sweep tube rated even higher with top-cap. 6L6GC guts are also the same as 7027A (600V). And so many g-amp makers have pushed the envelope, "all" quality g-amp tube suppliers use the better plate-stuff.

> you have to crank the gain and master volume nearly at max to get some decent volume,

That aint right. Even if the output stage is biased cold, the preamp and driver are able to SLAM it to 100++ Watts, which is enough to drive you from the room.

Compare voltages on ALL tubes to the chart. A lazy V1 will suck enough gain to make you work too hard even dimed.

Clean any insert, FX, or Aux jacks. Dirty contacts = lost signal.

Post on a guitar-amp forum where a lot of F-135 fixers hang. There may be some common fault.

I do NOT think you should mod it until it works as-good-as it did when new. It didn't suck, obviously, since there's tons of them around. It may not be today's cup of tea, true. But you can't improve something broken until you un-break it. And it is probably something simple.

I *might* consider an output bias mod. Fender's forever-fixed bias worked when Fender was the last major buyer of tubes and could demand well-matched crates. Today's tubes vary a lot more, why you are seeing very-low idle current. However it should PLAY LOUD just fine without that; bias is about smooth small sounds.

 

[zayance]

Thanks a lot PRR for the explanations...

Well Well Well, as PRR said, it was still weird that the amp wasn't delivering enough power...
And after checking all voltages, and they all seem to be fine ( pdf attached if curious), i said it's not possible, and all should be "ok"
So i just ran a small signal with my generator (50mV) and there is nothing wrong with the amp,
Seems like it's my freakin 2 cent guitar that was the problem (sold the others i had, i'm not much of a player no more), it will be burned over the week end, nights are still cold.... I feel like a prick  :-\  :-[, but anyway you live you learn as always...

Now the thing is the bias is still sitting at 5mA not 50mA, i'm using a small wirewound 1 Ohm resistor, and reading 5mVDC across it, so kind of weird, and don't know what to say or think about it...

I *might* consider an output bias mod. Fender's forever-fixed bias worked when Fender was the last major buyer of tubes and could demand well-matched crates. Today's tubes vary a lot more, why you are seeing very-low idle current. However it should PLAY LOUD just fine without that; bias is about smooth small sounds.

I'll do the AB763 schematic output bias mod i think, as you guys suggested and as i had come across when doing some search, i think it would be nice, he is anyway going to stick with JJ's i guess, so changing them will maybe not move the bias much in the future, and if he likes the sound when ran a little hot, then good.
He plays a lot, and so he wouldn't want to spend to much cash on tubes, but he said that after changing to the JJ's he lost a little bit of gain, and the sound was more "smooth" as you said than a more beefy sound, so that's why he asked me to have a look at the amp, even if i said i'm not a pro in that domain, i like challenges and it's always nice to learn by doing...
A little more delicate when it's old amps, you don't want to screw up on those things,...
And i knew that if a was really stuck i could ask over here for some help  .

Thanks a lot again guys, and sorry for the "prick" moment...
Back to the bias


EDIT: I had found this read about mods http://lenardaudio.com/education/13_guitar_amps_4.html