20W JCM800 - filter capacitance?

[Che_Guitarra]

I plan on building a 6V6 powered JCM800 to use up the DR transformers i've had on my shelf for the longest time.  Here's my base schematic:

I have a handle on most of the necessary mods.  But i've never built a solid state rectified guitar amp before, so i'm curious if I need to revise the filter capacitance values in the B+ rail as I drop the circuit from 50-ish watts to 20-ish watts.


Hmmm...

 

[andyfromdenver]

those values will work for your design
best wishes with building it!
Andy

 

[abbey road d enfer]

But i've never built a solid state rectified guitar amp before, so i'm curious if I need to revise the filter capacitance values in the B+ rail as I drop the circuit from 50-ish watts to 20-ish watts.

You don't need 100uF as 1st smoothing cap; 50uF is adequate. You don't need duplicated (series-connected) rectifier diodes if you choose the right PIV. 1N4007, with 1000V PIV is adequate.
470V on 6V6 is a lot; even Fender didn't dare do it!

 

[CJ]

what is he ratio on the output ?

drop screen to 380 V. bias to minus 38,  wait for arc-over,  ???  hold your breath if playing live,

plate function in a pentode is to collect electrons only, not attract them,  screen function is to  set e field to accelerate electrons into plate, so screen voltage controls plate current, 

you have a hi z output circuit with 460 volts on the plates, gonna need a hi z output, might not sound good, better change pwr xfmr to real world 6V6 specs, ie, 400 on plates,  there is a reason why nobody has built an amp with 460 on 6V6 tubes, why not learn from them? 

50 uf is better, cap forms resonant circuit with output transformer,  so you have a tuned circuit,

 

[Gene Pink]

... You don't need duplicated (series-connected) rectifier diodes if you choose the right PIV. 1N4007, with 1000V PIV is adequate.

Well, maybe, but it makes me uncomfortable. If that winding peaks out at +470V under load to top off the capacitor, 1/2 cycle later, it will go to -470V and a bit more. That is at least 940 PIV across the diode, not much margin.

Gene

 

[Che_Guitarra]

Thanks guys.

Just to reiterate:

- PT = Deluxe Reverb spec.  330-0-330 / 660V CT @ 120mA (Classictone 40-18017).
- OT = Deluxe Reverb spec.  20W, 6.6K (Classictone 40-18087),
- Choke =Deluxe Reverb spec:.  50mA,  4 henry (Classictone 40-18040)

I've had this transformer set sitting on the shelf for nearly two years now... time to put them to use.  But my tastes have moved on from wanting a Deluxe Reverb.

The plan was to drop the B+ voltage back to GZ34 territory via Zener diodes.  There's a few build threads to follow on applying Zener diode strings, but if you guys know exactly how to approach this - i'm all ears

The other option is to stick to GZ34 rectification... but that's an 'if all else fails' last resort... the sharper the response, the better the amp will suit my playing style.

 

[Che_Guitarra]

you have a hi z output circuit with 460 volts on the plates, gonna need a hi z output, might not sound good, better change pwr xfmr to real world 6V6 specs, ie, 400 on plates,  there is a reason why nobody has built an amp with 460 on 6V6 tubes, why not learn from them? 

I've been thinking about this for a few days.  I don't want to build an amp that measures power tube lifespan in minutes rather than days/weeks/months, so i'm starting to tilt toward GZ34 rectification.  I'll also be loyal to JJ 6V6s... which i'm told are not real 6V6s... but I don't care - they sound great (regardless of their true kind), they're tough as nails, and I already have a bunch of them in my gear kitty.

SS rectification was my desire because i'm not a huge fan of vintage-style sag (overly rounded transients, bass response wooliness), but the reality is, this type of sag is only likely to be a problem with a GZ34 if I plan on diming the amp to eleven at all times - my most used spectrum is dirty-clean to grit, so a GZ34 should be just fine.  And I think a 2204 front end should  modernise an overly vintage amp response.

So it looks like i'll be using a fairly traditional Deluxe Reverb power section, mated on to a 2204 front end.  Does that mean I best stick with the prototypical 2204 50+50uF caps up to the PI, or will GZ34 rectification require me to tame these values back a bit?  I'm used to implementing a trickle down cap value system from plates to screens to PI to preamp... i'm not sure how to adjust subsequent values if I want to stay true to a DR power section, and a 2204 front end.

 

[CJ]

those pwr xfmr ac voltages will eat that gz34 for breakfast, lunch and dinner.

yes, they say 1500, but in real life...

here is a chart that might help you out in trying something better>

 

[Che_Guitarra]

Thanks for your input CJ.

Now i'm completely lost.  I have a transformer from a highly regarded manufacturer, clearly marketed as Deluxe Reverb spec, that is too hot to use in a Deluxe Reverb? 

Edit - reading the 40-18016 spec sheet (same as 40-18017, but 120V only), it says in the bottom left corner B+ voltages with 5AR4 = 413V (@120mA), less with a 5Y3, even less with a 5U4.  http://www.classictone.net/40-18016.pdf.

Hmmm...

 

[Che_Guitarra]

Can I just ask - where did the 470V B+ valuation come from?

I plan on building a 2204 front end grafted onto a Deluxe Reverb power section - I think there may be confusion that I want to put 6V6's in a regular 50W 2204?  (oh I see now - DR transformers = Deluxe Reverb transformers).  I hope that clarifies my query.

Classictone's spec sheets suggest I should expect a B+ of 413V through a GZ34, via their Deluxe Reverb style power transformer.

Scratch that schematic in the OP:  Here's a rough-and-ready, v1.01 schematic:

 

[abbey road d enfer]

Can I just ask - where did the 470V B+ valuation come from?

It comes from the schemo in your OP, where B+ is labelled at 468V!

Classictone's spec sheets suggest I should expect a B+ of 413V through a GZ34, via their Deluxe Reverb style power transformer.

You suggested replacing the valve with a solid-state rectifier.

 

[Che_Guitarra]

It comes from the schemo in your OP, where B+ is labelled at 468V!

You suggested replacing the valve with a solid-state rectifier.

OK - my bad - that is a straight up JCM800 2204 circuit.  I was looking to modify 'that' circuit to suit a 2204 built with a Deluxe Reverb transformer set; PT = 330 - 0 - 330 / 660V CT @ 120mA.  OT = 20W, 6.6K.

But Deluxe Reverb transformers are ubiquitously paired with a GZ34 rectifier, and i'm not finding many proven SS rectified 6V6 circuits to take cues from... thus I was looking for help.  *In particular* how I should adjust filter capacitance to my plans...  6 x 50uF caps in the B+ seems like an awful lot for a 20W amp.

 

[abbey road d enfer]

OK - my bad - that is a straight up JCM800 2204 circuit.  I was looking to modify 'that' circuit to suit a 2204 built with a Deluxe Reverb transformer set; PT = 330 - 0 - 330 / 660V CT @ 120mA.  OT = 20W, 6.6K.

But Deluxe Reverb transformers are ubiquitously paired with a GZ34 rectifier, and i'm not finding many proven SS rectified 6V6 circuits to take cues from... thus I was looking for help.

Well, 330x1.414 is about 460, so the actual B+ will depend on the transformer sag, which in turn depends on the idle current. I can't predict without additional data, but I would think B+ to be significantly higher than 400V, which is a lot for 6V6's. It doesn't mean it won't work, but finding a safe operating area takes some experimentation and caution.

  *In particular* how I should adjust filter capacitance to my plans...  6 x 50uF caps in the B+ seems like an awful lot for a 20W amp.

The value of the 1st cap does not change much the B+ voltage, unless you use a too low value that would result in unacceptable ripple. Indeed you don't need 100uF as in the Marshall, but increasing from 16uF on the Deluxe Reverb to 50uF for a SS-rectifier is not heretic. The combination would result in a tighter response. The rest of the caps in B+ are there to improve hum rejection; the higher the better, within limits.

 

[Che_Guitarra]

Brilliant.  Thanks A.R.D.E.    That SS filtering guidance is exactly what I wanted to know.

Project 20W 2204 is a step closer to realisation.

As for the PT being too hot; i'll have to take my chances.  Classictone suggests to expect a B+ voltage of 413V via GZ34 rectification... add 20-ish extra volts due to SS efficiency gains - that's still safe territory for JJ 6V6s.