Marshall JTM45 clone with 30W of power.

I want to build a JTM45 clone, but the power output seems a bit high for my needs, it would be better to have 30W like later JTM45 had.

So the question is what kinds of modifications should I do, to get about 30W of power?
I guess I would have to changed the power tubes and the rectifier? Which would be suitable for that kind of power?

Also the output transformer to have about 30W instead of 45W. I guess it's not a must, but otherwise it would seems like a waste. OT saturation is certainly welcome.

Personally I think the only good way to drop the level on an amp like this is to use a dummy load. A master volume wouldn't work well in this case because it will interact poorly with the presence control. Otherwise, it seems like a lot of trouble to go through for a 15W drop.

Lower B+ plate voltage will lower the power.  Use transformers and output stage of something with the power you want.  Some of the Trainwreck clones would be one option.

Since 100W is only twice as loud as 10W dropping the output by 15W is actually a pretty insignificant difference.    WIth that in mind, personally, I would just build it as is, but it's your build so you may think differently.

I would drop the B+ (some dropping power resistors)  and bias the finals to run the desired power.  I think a pair of kt66 would be similar to el34 or 5881 - in my book, that would be good for a clean 30W.

100W is pretty crazy in an amp unless you are a rock star or a young e-guitar player!

30W is a good choice in my opinion - I have some el34pp in my set of amps - it's the 'class act' of the bunch.

Even better ...  10W of clean 6V6 power is my #1 fav - push-pull with cathode bias to some more of  'class A' (dropping the output power a couple of watts from 14 to 10  ... ).  Add a variable neg feedback to further tame the beast at max power as desired  .... 

I definately prefer a 'hifi' type of output transformer  as opposed to the small-ish ones often used in guitar amps. Give me a hefty hi quality traffo any day .. With proper 'dials turned to 10'  power tube distortion from modest finals,  the sound is record ready.

Anthon said:

So the question is what kinds of modifications should I do, to get about 30W of power?

Click to expand...

You could simply go to cathode bias. Look at how the Semer Zodiac Twin went from 30W to 50W, by changing from cathode bias to fixed bias.

However, as it has already been mentioned, going from 45 to 30W is a mere difference of 1.8 dB.
Have you considered the possibility of cloning a Marshall 18W?

First thing that came to mind was cathode bias , you could set it more for class A ,than A/B , more standing current , less power and a more compressed, less dynamic sound when overdriven .  20 watts tube like the Marshall 20 w Abbey mentioned would also be worth considering , for smaller venues these work great , if you need more output volume you could run a 2x12 or even 4x12 off it.
I have a 30watt Selmer too ,2xEL34 in cathode bias ,driving a big 15 inch Goodmans ,Gz 34 ,it dates from the late 50's , the range of tones from it is awesome ,especially when you push it to feedback , think early Pink Floyd with Barrett on guitar .

Cathode bias would work,  but it's a more noticeable change in tone and feel of the amp when going from fixed to cathode bias.  So it really depends on the sound you are going for.

use  75 ohm 10 watt cathode resistor

combines fixed with self bias,

might need to restructure voltage divider resistors in bias circuit to lower negative volts

I have been looking at these VVR circuits with some interest.
Might be something you are interested in.

http://www.tonegeek.com/musicgear/amps/variable-voltage-regulator-vvr.php
http://guitarkitbuilder.com/install-variable-voltage-regulator-guitar-amplifier/
http://docplayer.net/62405575-Vvr3-variable-voltage-regulator.html
http://revolutiondeux.blogspot.com/2012/06/voltage-scaling-in-amplifiers-power.html
https://www.youtube.com/watch?v=SET_Q10vsJA

Those VVR circuits are interesting!

Thanks for that, it's the first time I've seen a versatile module like that.

..

Here's a pic of my finished small amp head 

It could have used a VVR module! It has JJ 6V6S finals with 400V B+ and variable nfb for 11W  clean  (1% with >80dB SNR)  or 15W  crunchy (5% with low nfb)

Uses old UTC in the line amp, and Sansui hifi amp output traffo from the 60s.

Thanks for all the input!

Cathode bias seems like a good option, I like how it sounds over fixed bias.
Getting inspiration of Trainwreck is also something to consider.

But I thought Marshall switched to 30W in their JTM45 at some point? I wonder how they did it.

15W less might not be that much, but I've already built 45W and 22W (and also 15W and 5W), so I thought I go somewhere in between this time 

Run output tubes in triode mode.

+1 the triode mode - it works really well if one is OK with the basic idea ...

In light of the neat afore-mentioned  'VVR' module (for simul-control and regulation of bias and screen voltages),  I'd like see a module that can do simultaneous control of the triode - pentode - ultralinear  switched modes with commensurate bias changes  ....  to run each mode at it's own happy optimum.

ie. I have done simple triode-pentode mode switching on my moderate powered instrument amps,  but haven't taken the time to be clever about *also* adjusting the bias when switched. 

I bias them up the way I like for pentode (or ultralinear) mode,  which is the most frequently used;  when switched to triode mode, it works fine but is not optimally biased - like I would normally attempt to do  -  so definately some scope for a neat module, VVR style, to do the job properly!

Something like switching in a 'fixed bias' portion to add to the 'cathode bias' portion  of the total bias  .... kind of thing

....

So sure, el34pp good for conservative 30W drops down to prolly 22W or something like that. Less THD and more refined sounding. Very nice.

I did that on my parallel SE amp head with 2x6550 doing 22W in ultralinear  or 15W in triode mode  (120ma standing current).

I do use the triode mode most on that amp, despite significantly degrading it's already appalling power efficiency  8)

Maybe a good plan would be to bias the output tubes class A with cathode bias, then when the higher power ranges are required switch in some extra negative volts at the grids , having to adjust a cathode bias resistor is awkward , although a two pole  switch can be used to switch the the screen grid to plate and cut in an extra cathode bias resistor across whats there already ,thats how I did it on my single ended EL84 amp , its a modified Peavey Nano valve . Ht is a bit high for a single ended Trioded EL84 at 275 volts , it hums a little bit in triode mode.

The other thing I was thinking was tubes probably like to be set for proper current in a circuit and left alone , there might be a penalty to be paid for switching the standing current in a pair of tubes , as well as the less than optimum load , it might effect the lifespan of the tubes . There are other ways to vary the Ht volts off your rectifier than using sillicon and transistors though , if you use a big choke and a switchable first filter cap  ,somewhere between 5 uf and 50uf ,gives a whole range of Ht volts ,downside is the choke will be as large as the output transformer and have to carry the full HT current as well as the ac current , your varying between more choke input filter with small capacitor ,to CLC with larger first filter caps .A 10 or 20 hy choke also makes your ht very quiet.  Simlpy having a tube /sillicon rectifier switch would instanly give two choises of Ht volts , with an attendant difference in tone .

I think Seymour duncan once made an amp with a range of switchable powers , I forget the name of it ,but the circuit could give up some inspiration for what were doing here . I'll see if I can find it .

good stuff here ,

https://www.youtube.com/watch?v=vfVDLqVMec4

the Seymore Convertible ,

schematic and manual is here ,

https://elektrotanya.com/seymour_duncan_convertible_100_manual.pdf/download.html

good stuff here ,

https://www.youtube.com/watch?v=vfVDLqVMec4

the Seymore Convertible ,

schematic and manual is here ,

https://elektrotanya.com/seymour_duncan_convertible_100_manual.pdf/download.html

Click to expand...

Jesus 575 volts on the plates.  My old Marshall 100W (1970) had 565 volts and had a clean sine wave at 160watts rms.  This amp would have to be an ear and arm breaker when turned up.  I remember playing one years ago but had the attenuators turned down and don’t remember much except there was a possible microphonic tube in it. Anyway anything to save the ears and get the tone is good with me.

Regarding the mode+bias 'switching' - normally I use a heavy-duty dpdt rated at something like 240vac 10A or 15A, depending on how high the the B+ and how much current is in play ...  so the switch is either 'moderately large' or 'quite large'. 

Best to be conservative when switching around +400V - seperation of switch terminals is important for safety I think, as well as a very robust mechanical.

Fortunately there are affordable such types on eby. 

Going beyond dpdt is a different matter - I can't think of any offhand, so doing a nice 'switching in/out' of a bit of extra cathode resistance gets awkward if one is dealing with pairs of final tubes. 

Perhaps one could use a large dpdt and a small seperate dpdt to do the cathode circuit switching - effective but not too elegant!

For that reason, making a module becomes more attractive - something with a relay or two perhaps ... 

Then again it does get quickly out of hand  ;D  One reason why I and many others, have several amps at hand at all times!

Interesting discussion has led me to look at reducing the screen voltage to control the power dissipation.  I have an old Standel I’ve been carrying around for years.  It has a 6BQ5 PP output stage and used early SS modules for preamp.  Those preamp modes were removed from the amp due to blowing up before I got it.  Anyway I’m thinking about an ac30 or 1974 Marshall 18 watt. .  The power amp worked and puts out 20 watts .  It uses a voltage doubler with 420 volts on plates of a 6bq5.  It then runs the screens at 150volts so tube doesn’t burn up.  So reducing screen voltage is a way to adjust the tone and the powers of the amp.  I see this in some bass amps and believe that was the original design of the Standel head.  So I need to just install a preamp tube and see how it sounds.  I have Hammond 270fx as a backup 275-275 with 6.3 and 5 vac for a backup if needed.  The output transformer is big for a 20w 8k to 8 ohm so should be fun. 

Good info here for screen grid affect on plate and power .

http://oestex.com/tubes/screens.htm

Very interesting, thanks ....