Tweed Bassman / Deluxe w/ reverb. any opinion?

[loudthud]

A couple of quick observations. A 27K feedback resistor from the 16 ohm tap? Usually it goes to the 2 ohm tap.

The .02uF cap from plate to cathode of the trem oscillator. Seems kind of high for a tube with a 470K load resistor. Try it, but remove if it won't oscillate.

The reverb arrangement will feedback on itself so you have to limit the amount. There is a Champ (Champ12?) that used the speaker output to drive the reverb and there was a trim pot to limit the reverb. I like the mix resistors at the PI input better. IMHO YMMV

[tubeswell]

A couple of quick observations. A 27K feedback resistor from the 16 ohm tap? Usually it goes to the 2 ohm tap.

Thanks loudthud - the NFB from the highest tap will still work, it'll just be another few ohms in the NFB loop (from the extra length of secondary winding between where the speaker is tapped and the 16R tap) when any of the other taps are connected - I did this with a 5F2A last year and it worked fine - allowed me to get NFB with any tap I plugged in.

The .02uF cap from plate to cathode of the trem oscillator. Seems kind of high for a tube with a 470K load resistor. Try it, but remove if it won't oscillate.

Thanks for the tip. I just copied the LFO from the 6G16 circuit (but now I know what to do if it won't work ). I thought it should work because the HT voltage in the 6G15 schematic is 440, vs 430 in the 5F6A schematic (not that schematic voltages mean a lot, but I was using them for a guide)

The reverb arrangement will feedback on itself so you have to limit the amount. There is a Champ (Champ12?) that used the speaker output to drive the reverb and there was a trim pot to limit the reverb. I like the mix resistors at the PI input better. IMHO YMMV

I was wondering about the possibility of the reverb feeding back on itself while I was mowing the lawn just before (the weather is so nice here this W/E) and I wasn't sure whether I had drawn this the way MWJB was suggesting because of that. I realise the mixing resistors (shaded green in the previous schematic) wouldn't have this problem. (But I was wondering whether there would need to be another driver stage in front of the PI if I did it that way). Would the mixing resistors attenuate the signal too much?

There is still the Hoffman style 18W stout reverb add on option to consider. I must schem that up.

[erndawg]

the NFB from the highest tap will still work, it'll just be another few ohms in the NFB loop (from the extra length of secondary winding between where the speaker is tapped and the 16R tap) when any of the other taps are connected -

Actually, the effect of NFB is directly proportional to the amplitude of the OT's secondary. The voltage at a 16 ohm tap is substantially greater than that of an 8, 4, or 2 ohm tap. The higher the voltage, the greater the feedback, the cleaner and tighter the overall response. The output stage thus becomes less dependent on power tube differences.

[tubeswell]

Well that's interesting - so it works 'even more'. When I did this on my 5F2A build the NFB still sounded good. I suppose I could get more complicated and have the secondary taps hooked up to a multi-pole switch (with the NFB going to the output socket tip. I have a few 3-pole 4-throw rotary switches lying around (which I could wire in parallel for insurance).

[erndawg]

Changing the value of the feedback resistor does the same thing. You end up with a different voltage divider ratio. That's why some builders install a pot with a series resistor in place of the feedback resistor. They can then dial in the NFB they want.

[tubeswell]

Version 8 - Standalonish reverb circuit?

On this one I have gone back to keeping a more traditional tweed front end and am taking the reverb from after the channel vol mixing resistors. I figured that this way the non-reverb part of the circuit stays more authentically bassmanish (if this abomination can be regarded in that way).

I have a spare triode, so I am tossing up whether to put it in front of the paralleled 12AT7 , or putting it as a CF stage driving the reverb tone control.

Mr Hoffman's site suggests to get a deep fendery reverb you need a couple of stages before the reverb driver, so that was the idea behind this version. Does anyone have a view on that?

I am not sure about whether I need a bit of padding in front of this extra triode (see red resistor labelled 270k for want of a bit of a guesstimate). I wondered whether this may be necessary to stop the input impedance of the direct coupled pair stage being dragged down (but I don't know if that's really necessary). I am guessing that the extra reverb triode probably needs a 1M grid load resistor - so I put it in.

Since I now have 6 triodes off the 10uF pre-amp decoupling cap, I wonder whether this needs more filtration? (20uF Sprague Atom maybe?) Or should I split the power supply rail further back and have (say) another 10k after the PI plates with another (say 20uF) filter cap? (or will that possibly introduce another potential source of hum?)

I also took out the level control after the recovery stage and just left a simple tone control there (is that the 'best' kind of one-knob tone control for this type of hook-up? - I dunno). I was a bit concerned about how much current a 5F2A-style vol and tone would eat off the reverb recovery stage here. (or I could have a 6G15 type tone control here, or use the extra triode I have put at the front end, to drive a reverb recovery with a CF direct-coupled pair to drive a 5F2A style vol and tone stack instead, and then keep the 2 x 270k mixing resistors in front of the LTP)

I have made the level control into a 250k mixing pot in front of the LTP.

I figured if I made the chassis 2 feet long I'd be able to fit all 11 knobs on top with the sockets and switches etc, and the 6 pre-am tubes and three octals underneath. (I'm envisaging that the amp will have 2 x 12" Greenbacks side by side instead of 4 x 10", so it'll be long and low and maybe more like a bluesbreaker/JTM45 with reverb and bias-vary trem).

Further critique welcomed

[tubeswell]

Ver 8C

More tweaks

1) impedance selector on OT and reconfigured NFB

2) reverb tone control based on 6G15 (Edit - I've just realised that the coupling cap from the reverb recovery stage oughta be something like 0.1uF - oh well :-) You might say that this tone control is superfluous if it is in front of the main tone stack, but I was thought leaving it there might help the fine tuning. (Hell its gonna have 11 knobs anyway right?)

3) reverb return now before the direct-coupled pair - do you think the 250k mixer is adequate? (Or should I do some kind of voltage divider reverb level control and a more traditional 3M3 bypassed with a 10pF cap here? There is no driver stage immediately preceding this tho', so I am not sure) Also same question on the red 270k resistor on the reverb circuit tap as last time. Is 270k enough here to stop the reverb feeding back on itself? Does it need a small cap across the 250kN pot as well?

4) reconfigured power rail with split supply and extra filter cap separating pre-amp and reverb pre-amp and recovery stages. Will this work better d'ya think?

[tubeswell]

Bassman with 'verb & trem V10

(many many thanks are particularly due to my M8 loudthud for the most helpful comments and feedback leading to this revision) (and thanks are also due to MWJB for help in the initial stages and pdf64 for the germ of an idea)

Dare I say the 'final' topology is here? (there are still some component values to firm up)

Only question is how much will the extra 4 little bottles load down the B+?

If somebody can't help me with a theoretical account of that, I guess I'll find out by building it.

[txstrat]

Did you intend that ALL the signal goes through the verb section?
I'd rather stuck with the version where the MIX pot dials in the verb at a certain rate.

[tubeswell]

Did you intend that ALL the signal goes through the verb section?
I'd rather stuck with the version where the MIX pot dials in the verb at a certain rate.

No take another look

The vol mixing resistors have two signal path options (controlled by the 250kN mix resistor). The dry option puts the signal through a CF, (in the vein of a 6G15 dry/wet mix) so when its dialled to the dry side, it basically preserves the signal that would've otherwise come from the mix resistors, but only now its better, because there is a unity gain low impedance buffer/driver into the direct-coupled pair.

I wonder what size mains fuse I should put in this?

[txstrat]

No take another look...

Yeah, didn't recognize it at first glance.

I wonder what size mains fuse I should put in this?

I'd say it depends on the PT. What values will the secondaries have?

[tubeswell]

I wasn't going to fuse the secondaries (just use a 1N4007 on each side of the rectifier winding before each rectifier anode).

(I thought the size of the mains fuse was somehow related to the current draw on the secondaries??)

[txstrat]

Yes, that's right multiply volts with amperes (for each winding) and you get VA. Sum up the VA values and you get the whole VA of the secondaries. Divide that by the mains voltage and you get the primary amperage. Take the nearest value and multiply by two.
For instance
PT:
300v - 200ma = 60va
6.3v - 4a = 25.2va
50v - 80ma = 4va
89.2 va altogether

89.2va / 110v = 0.81a

Match to 1A and multiply by two = 2A mains fuse.

[tubeswell]

Excellent thanks txstrat. So 2 Amps it is

I don't suppose you know how to calculate HT winding voltage loads based on the number and type of tubes?

I have 7 x 12A_7 preamp tubes and 2 x 6L6GC, and I was wanting to figure out how much the voltage would drop once it was all loaded up. The PT I already have has an HT winding of 325-0-325 200mA. (I could get another one made up without much ado - just wanting to know if it was worth it) (I see the vibroverb uses 380-0-380 on the HT winding for 2 x preamp tubes and 2 x 6L6GC)

[txstrat]

Some Hammond replacement transformers (e.g. the 274BX) have a secondary HT of 375- 0-375 at 175ma while the 6.3v windíng has 6A (at least enough for 6 or 7 preamp tubes and two 6L6 even with some spare amperes). I would say the 200ma of your trannie should be plenty. What about your heater winding?
The vibroverb 6G16 uses six preamp tubes and two power tubes. You're gonna add one more 12AX7 (1.2ma plate current) and that should be fine.