4 Tube Amp From Scratch

[TubeNoob]

Hey all, I'm new to this Forum but I've heard a lot about it, so it's time to join!

I'm converting an old radio to a guitar amp and drew up this schematic based on 3 things: Original radio and its parts; Kalamazoo Model II (tremolo circuit); and Fender Harvard (since it uses a 6AT6 as preamp tube as did the radio).

This is my first design from scratch, so i'm sure there are problems with it. I'm really just kinda throwing myself in the deep-end with it. Would love to get some input from the pros.

I'm not a 'total' TubeNoob as the handle implies, (though close) - i have built a Fender Princeton and a Tube Filter so far.

Here's my schematic...

[loudthud]

Do you have a power transformer? Those type tubes, 50B5 and 35W4 are usually operated directly off the power line. Sort of a no-no these days because one side of the power line is connected directly to the guitar strings. Too dangerous. You will need at a minimum a 1:1 (2:1 outside the US)isolation transformer to make the amp safe.

The heaters are connected in series. Which brings up the point that all the heaters must have the same current rating. The 6AT6 has a 300mA heater and the rest have 150mA heaters. So to use that tube you will need a filament transformer or you will burn about 20 watts in a series resistor.

R17 1 Meg? That will never work. Maybe 100 ohms at 5 or 10 watts. R10, 80 ohms? That's not needed. There is no filtering for the B+ to the preamp. The amp will hum like crazy and may oscillate. The Kalamazoo tremolo might cause problems. There is no filtering of the LFO (Low Frequency Oscillator) signal so too much can bleed into the power tube. This may or may not be to your liking where the signal distorts in sync with the tremolo. There is no Intensity control for the tremolo which limits it's usefulness. C4 1uF? 500pF would be a better choice.

[TubeNoob]

Thanks a lot for the great input, LoudThud. Just what i was hoping for.

The radio has an 'autoformer', but I do have a regular transformer i can use if there's no way to safeguard it otherwise.

Regarding filament currents, i was aware of that and read about a trick by Fred Nachbaur i was hoping to implement where the 12ax7 'sums' two 150ma lines into one 300ma line. The way the autoformer did it was it supplied a 2nd line into the series after the 150ma tubes. (The 6AT6 is part of the original radio, as were the 50b5 and 35w4).

Changed those values you suggested - and thanks for pointing out the tone cap at .0005, i missed that from the Harvard schematic (1uf is the default value in my software).

Regarding the 80ohm/R10, that's on the original radio, too, so i figured i might want to include it, but if its not necessary - cool, i removed it.

I chose the Kalamazoo trem because i actually love the distortion in sync. I used to have one and really dug it.

Would simply adding a pot to the cathode line work for tremolo amount? (I added it to this updated schematic)

Also added caps to filter that B+.

Thanks again for your really helpful info.

[loudthud]

You are getting closer but... you need a dropping resistor in the B+ line between the power amp and preamp. Still problems in the tremolo circuit. Study the Vibro Champ: http://www.el34world.com/charts/Sche...HAMP_AA764.pdf A little more complex than the Kalamazoo. See of you can combine the two.

[TubeNoob]

Ah...yeah on that last revision i screwed up some stuff on the tremolo part - I fixed that (it's now how it was on the first version i did), revision is attached.

Also, added ground to the Tremolo Freq pot, as on the VibroChamp. Seems just attenuating the signal to the preamp cathode does the 'intensity' job. Not sure what else i should copy from the vibrochamp.

As for the dropping resistor, I added R10 at 10k. Is that what you were suggesting and does that sound like a reasonable value? (The VibroChamp uses a 10k).

Thanks again, LoudThud!

[loudthud]

You still have a problem with V2a. There needs to be a resistor to ground from the cathode for the tube to bias properly. Something around 3.9K should do the job, but it depends on how much B+ that stage gets. Also, the 4.7K (R2) on the V2b cathode may need to be adjusted for the tube to oscillate. R8 is probably not optimal for the 6AT6. The plate voltage will be low and that will cause the pots on you guitar to be scratchy. Again, you will have to experiment with the value but start with something around 3.9K or 4.7K.

You don't need both C13 and C14. One will get the job done. You have them connected in parallel. 10K for R10 should work good.

[TubeNoob]

Wow, your 1,500th post! Nice...!

So, I changed those values and removed the extra B+ cap.

I had R8 (6AT6 cathode resistor) value at 1500 from the Harvard schematic, but changed to 3.9k per your suggestion.

I moved the cap and resistor from the Trem tube up to V2a (as per the Vibrochamp). Will the Trem tube cathode need it's own R/C to ground? (Vibrochamp doesn't have it on stage 2 in the Tremolo part).

Updated schematic attached.

You're a great help, thanks a lot!!!

[loudthud]

Wow, your 1,500th post! Nice...!

I had R8 (6AT6 cathode resistor) value at 1500 from the Harvard schematic, but changed to 3.9k per your suggestion.

I moved the cap and resistor from the Trem tube up to V2a (as per the Vibrochamp). Will the Trem tube cathode need it's own R/C to ground? (Vibrochamp doesn't have it on stage 2 in the Tremolo part).

My goal is to have one post for every 10 of Enzo's!

I think I found the Kalamazoo schematic where you got the Tremolo circuit.

http://www.el34world.com/charts/Sche...amazoo_m2s.pdf

That tremolo circuit looks like it would barely work, but it won't hurt anything to try. With an Intensity pot, V2b really needs it's own RC to ground to stablize the operating point (bias). A big unknown in this case is how much B+ voltage there is in your amp. Most amps like yours have a B+ of about 150V when a 35W4 is used directly off the power line in the USA. I suspect you might be in some other country with a 240V mains and be using a 240 to 120 isolation transformer but that won't change much.

One issue will be if V2b (your schematic) will have enough gain to oscillate. There are two things fighting against each other. The 150K plate resistor would give you high gain but the low plate supply voltage tends to lower the gain. In addition, the coupling of the tremolo signal to the cathode of V2a is a little unpredictable. With R6 being 270K, V2a will have a lot of gain. It could be more than is needed to drive the 50B5. But with low current going through V2a, it's easy for the tremolo signal to modulate the guitar signal.

So, I suggest you go ahead and build the amp. I hope there is ample room in the chassis. Don't expect things to work perfectly the first time. There will probably be some resistors that will need to be changed to get the Tremolo working properly. Try to keep that in mind and don't make it too hard to change a resistor or two later (including the Intensity pot).

[mooreamps]

ah, guys !!! {tink, tink, tink}.......

no one sees something with S1 ?

-g

[R.G.]

ah, guys !!! {tink, tink, tink}.......
no one sees something with S1 ?

Yes, we see it.

However, did you ever think that *this* would be the time where you could redeem yourself a bit by explaining it to him in a non-self-aggrandizing, non-confrontational manner? You've shown improvement recently, in that your recent posts did not overtly tell us all again how very smart you are, and have not belittled others in an obvious way, nor expound on the wrong side of some technical issue. This is very good, and shows hope that you may be trainable, which was something I hoped for.

So, while you're on a good streak, go ahead and explain calmly to the poster what about S1 you see. Try not to refer to yourself, and try to use only technical explanations that can be verified by third party references. Also try not to imply that someone else was in some way deficient for not noticing it sooner.

I have great faith in your abilities if you can just concentrate on the goal.

[TubeNoob]

Nothing like a lively conversation about Tube Amps!!!!

So, indeed - S1 shouldn't be on the first stage, my bad. When i built my 5F2a i put a pot on the 2nd stage cathode cap so i can change presence, and it's a great mod that i use all the time.

Any problem putting a switch or pot on the 2nd stage cap on this amp, given that its cathode is being modulated by the tremolo? (I've added it to the revised schematic below)

LoudThud - so to lower the gain of V2a and make it easier for the tremolo to work I should increase the 270k/R6?

Added R/C to V2b cathode, per your suggestion.

I am indeed in a 240v country. I have a gigantic transformer that puts out multiple voltages (185v, 14v, 8v, and when combined, 220v), 80VA. Figured i'd move on to the power supply and heaters next...

[R.G.]

Nothing like a lively conversation about Tube Amps!!!!

Sorry about that. We're trying to get Gary use to using his indoors voice.

So, indeed - S1 shouldn't be on the first stage, my bad. When i built my 5F2a i put a pot on the 2nd stage cathode cap so i can change presence, and it's a great mod that i use all the time.

Actually, you may or may not want S1 on the first stage. That's up to you and what tone you want to get.

I don't know for sure that this is what Gary was seeing, but it's what I saw: S1 is set up so it will give a huge pop when it's switched, aside from the effects you may be trying to get. You may not be planning to switch it while the amp is on, but if you do, it could be a problem. As shown, it either switches in the cap or shorts out the cathode resistor to ground, which causes a massive change in the bias on the tube.

Instead of how you have it switched, you might want to put a large resistor - 100K or more - in series with the cap, and use S1 to short across the resistor. The large resistor lets the cap charge to the DC voltage on the cathode, and so the cap does not have to be charged up when the switch connects it in, and you don't get a sudden change in DC conditions.

At least that's what leapt out at me. But sometimes I miss things.

[mooreamps]

Sorry about that. We're trying to get Gary use to using his indoors voice.


Actually, you may or may not want S1 on the first stage. That's up to you and what tone you want to get.

ok......"sigh'''''' I'm using my best "indoors voice".... What I was seeing was ; the switch as drawn ; you toggle that switch and it puts a ground on the cathode of V2a....

-g

[R.G.]

ok......"sigh'''''' I'm using my best "indoors voice"....

You are, indeed. Good job. I knew you could if you concentrated.

What I was seeing was ; the switch as drawn ; you toggle that switch and it puts a ground on the cathode of V2a....

Yes. That's right. Very good.

And next - that's bad, but ... why?

And then:
- What problems would we expect from grounding/ungrounding the cathode into a cap like that?
- What might be a better arrangement suiting the needs expressed or implied by the original poster?

Go for it Gary. You're on a streak!

[mooreamps]

You are, indeed. Good job. I knew you could if you concentrated.


Yes. That's right. Very good.

And next - that's bad, but ... why?

And then:
- What problems would we expect from grounding/ungrounding the cathode into a cap like that?
- What might be a better arrangement suiting the needs expressed or implied by the original poster?

Go for it Gary. You're on a streak!

No, I'm not... You call this "a streak"? It's just nothing more than a casual observation..

I can think of several "better arrangements", but I'm not doing that anymore.. You want better arrangements, then cut the sarcasm first, and then ; maybe ; I'll think about....

-g