First tube amp project. Schem questions

[Mostro]

Hi people, first post here, I have been participating in various other forums, mainly DIYstompboxes, but I´ve caught the tube amp fever...

I´d like to ask for your help/insight/experience on my build project, the D´luxe Princess. This is my first tube amp build, I have previously made a tube preamp, but that was on a PCB, so it doesn´t really count (...).
I got a set of transformers for a 5E3 Deluxe, so I went from there as a starting point, knowing that it would work well to power two 12AX7´s and two 6V6´s in PP. What I did was use the preamp section of the Princeton AA1164, removed the vibrato and reverb part, and "glued" it to the 5E3 Deluxe power amp. since I had a triode left, I added another gain stage with volume control. By the way, my target result sound wise is clean sound with humbuckers at low gain, and crunch/dist at high gain. Then I added some things such as an Fx loop, heater elevation, bleed resistor, middle control, screen resistors, changed rectification to diodes, feedback and an external speaker jack (this will be a combo).
So I finished the schematic, here it is:

[ATTACH]4361[/ATTACH]

Later I´ll make the layout.

Now the questions:
1) First, will it work?? Have I missed anything basic?
2) Since I changed the rectification to diodes the voltages will be higher than with a recto tube, when I calculate the B+ after the diodes (325 x 1.4) it gives me 455V. I know that under load the voltage will drop some, but I think it´s too high for the 6V6´s, and also for the filtering caps, if I use 450V ones. I know that in Fenders the 6V6 is used above their specified plate voltage without problem (I think around 400V usually), but I don´t want to burn mine. So I put R27 after the diodes, to drop the B+, but I´m not sure what value should it have and what wattage rating. Anyone can help?
3) What do you think about the FX loop? Is that OK, design wise? I´m not sure about what the signal level and impedance will be at this point. Too high?
4) What about the heater elevation resistors, are they OK in value and wattage?
5) I upped the value of the feedback resistor, so as to get less feedback. will that work?
6) I added screen resistors for the 6V6´s although the 5E3 schem doesn´t have them. Will there be a problem with that?
7) What do you think about the speaker jacks connection? is it OK (the idea is that when you plug an external speaker, the internal one is disconnected)?
8) What do you think the result will be, sound wise? a "only dirt" amp? a clean machine?

Well, if there´s anything else you see that deserves comment, please do so. I know, it´s a lot of questions, but since this is my first build, I have zero experience, and that brings anxiety... I also know that some of my questions can be answered by doing the math, but I´m just learning about that, and usually my calculations end in total disaster...


Thanks for reading, for your time and your help.

[mooreamps]

R-27. I would start with 100 ohms, 5 watt.
R-28. I'm not sure you could source a 1 meg ohm at 5 watts. But I've seen values between 270 K up to 390 K in that range. If so, you may need to drop the value of R29 accordingly.
Interesting approach to the stand-by switch.
Otherwise, it looks ok to me.

-g

[Albert Kreuzer]

R28: For a 1M resistor to dissipate 5 Watts you need a voltage of 2236V across this resistor
With 450V, a 1M resistor will dissipate .2W, so a half Watt R will be enough.


Cheers,
Albert

[Mostro]

Thanks people for your answers, I´m getting a clearer picture.
Now, some people have said to me that the FX loop is not in a good place, that in that place of the circuit there´s gonna be too high an impedance and too much signal excursion. Would it be better to place it between the 3rd gain stage and the phase inverter? Or, I´m starting to think that it would be better to convert the 3rd gain stage into a cathode follower and put it after that... would that work?

Thanks

[stingray_65]

BF tone stack

It is my understanding that the black face tone stack you are using has very high insertion losses.

I was wondering if changing R4 and R8 from 100k to 68k would increase the preamp gain enough to compensate for the loss.

LOL its more of a question than a suggestion.

Ray

[tboy]

...changing R4 and R8 from 100k to 68k would increase the preamp gain

Lowering the value of those two resistors would decrease the gain. The BF preamp is already designed to make up for the loss in the tone stack.

[Mostro]

Ok people, I´ve reworked the design.

[ATTACH]4362[/ATTACH]

- Added a tube rectifier. I also added R27 and R30 to that, according to some things I read. Are they OK?
- Upped the first filter cap to 33 uF. Will it affect the recto tube?
- Eliminated the feedback loop. It will make the amp a bit more dirty, right?
- Added a switch to the third gain stage, the idea is to have more flexibility, from a "fender" type clean without the third gain stage to a more "marshally" JCM800 thing (I´m referring to the amount of distortion, not to the tone) with it in the circuit.
- Moved the FX loop´s location. I think here it will work better, at least in the sense of having more of the final sound at the send. If it´s not useful for interfacing with effects because of impedances and levels mismatch, at least I´ll have a preamp out and a power amp in...

Opinions?

[stingray_65]

Lowering the value of those two resistors would decrease the gain. The BF preamp is already designed to make up for the loss in the tone stack.

I stand corrected

Ray

[Enzo]

Why not eliminate that heater elevator stuff and just connect the heater CT to the output tube cathodes? A very old trick. Or just ground that center tap.

Why the fancy bleeder switch circuit? Just wire the bleeder across the filter cap so it is always there? We are now relying on this switch for safety.

You can reduce the negative feedback, you can make it variable and call it a presence control, you can even remove the NFB altogether. is there a reason you have chosen to do this?

The concern with your loop is the signal level, in my view. A line out should have a line level signal - a volt or less.

[mooreamps]

Hmmmm. Most layouts I've seen, the "boost" switch was just tied to a cap to increase the low end a bit. Here, it's actually tied to a gain stage making it a "real" boost switch. Very good . . very, very, good . . . .

I do have a couple of thoughts...

pin 1 of V1b ; add in series a 22K shunted by 100 pF. This will keep the gain of the tube inside the range of audio frequencies, and not RF frequencies... Same with pin 2 of V2a.

C6 and C9 ; take them down to 10 nF to keep the chords from sounding too muddy. Single notes you may not hear any difference. Then, C10 ; bring that one up to 1000 uF. That helps the power tubes breath easier on the low end.

R19 and R20. I use different values for them, but what you have there is fine. Just make sure resistors you use for your grid stoppers, try to mount them directly on the tube sockets ; if possible. It helps.....

R30 and R27. Nice thought. It will give you some real nice old fashion compression along with the 5Y3. As an option, you could run a couple of 1n4007 diodes in series with pin 4 and pin 6 of the 5Y3. They will keep the power supply going if the rec tube gets an internal short. Not new tube, but old tubes that have been played and giged hard. . .

-g

[Mostro]

Enzo, thanks for the suggestions, they make sense. As you can see I eliminated the feedback loop in the last schem. I had a feedback loop to get a cleaner sound, but I don´t know if it will make a big difference.

Quote:
"pin 1 of V1b ; add in series a 22K shunted by 100 pF. This will keep the gain of the tube inside the range of audio frequencies, and not RF frequencies... Same with pin 2 of V2a.
-g "

Mooreamps, I don´t quite understand... you mean just before the grids of V1b and V2a (pin 2) and with the cap in parallell with the resistor?

Thanks

EDIT: about C10, you mean 100 uF I guess, or is it really 1000 uF?

[Mostro]

OK, here´s the latest schem revision. I changed the heaters reference, bleeder R, R21 and C17 location, added R28. What do you think?

Thanks for the help!

[mooreamps]

Mooreamps, I don´t quite understand... you mean just before the grids of V1b and V2a (pin 2) and with the cap in parallell with the resistor?

Thanks

EDIT: about C10, you mean 100 uF I guess, or is it really 1000 uF


Just like what you have for R21.
Yes, 1000 uF for 6V6.
470 uF for EL-84

-g