Tweed Bassman / Deluxe w/ reverb. any opinion?

[tubeswell]

Hi Peter

I drew a '1-channel bassman' something like that a couple of posts back but using a 6BM8. AFAICT it was pushing the boundaries of the PT I have for this build.

I do have a 6G15 clone I could easily stick in front of it (ground-loop issues aside for a moment). And my intention in building the reverb unit earlier on was that so I could use it with a variety of tweed style builds; where it does sound good (most spectacular on the 5G9 clone I built last year, because there is virtually no hum when used with that amp).

However I thought that if I could build a tweed style amp with reverb, that would overcome ground loop issues (which I still haven't managed to satisfactorily eliminate yet - mainly due to lack of proprietary ground lift adaptors in this country). Its just a matter of what might be the most workable location and configuration for the reverb circuit. 6BM8s are readily available, and although it dain't have that pentode sound, maybe I can use the spare triode from the last schematic as some kind of pre-driver stage before the reverb, which might help me get closer to a standalone configuration? Hmmm...

[tubeswell]

I'd suggest having the reverb re-injected after the tone stack, but have a tone pot on the reverb circuit, to EQ the wet signal.

Hi MWJB

How does this look?

The reverb circuit is located after the tone stack. (Why do you think this would be an advantage?)

I wasn't sure of the best type of tone control so I sort-of based in on the 6G15 one. (50kN with .01 to ground and 250pF to the signal injection path). Not sure if that will be effective or adequate, or if there is a more adequate set-up that will preform the job better here.

I eliminated the network of resistors near the level pot and changed the value to 250kN.

[txstrat]

Great drawing TW (think you know that already, huh? ) That's exactly what I would do.
I believe if you leave the bassman circuit alone as much as possible and add the verb right before the PI you wouldn't get too much of an altered tone CAUSED by the verb section. Just MHO.

[MWJB]

Hi Tubeswell,

"The reverb circuit is located after the tone stack. (Why do you think this would be an advantage?)"

Sorry, I only meant re-inject the reverb after the tone stack, not tap & re-inject after the tone stack. You could still tap the signal for the reverb after the first stage.

Not that this set up would not work, but tapping the reverb after the tone stack reduces the need for a tone control in the wet circuit.

If like before, you tapped the reverb signal after the first stage, then re-injected after the tone stack, the wet signal would not be affected by the tone controls. If say, it was dull & woofy there would be nothing you could do about it, as the amp's tone controls would only be affecting the dry signal.

[Victor Denance]

Hi there,

Yesterday I finally came to build that amp. It's much like Tubeswell's drawing on post #17, except the verb is simpler (no dwell or tone control), and te PI and power amp is much like a 5E3.

For some reasons I happened not to like the results. I had (still have) a very loud hum that increases when reverb knob is turned up which may have contributed to me not liking the tone (although all the rest was working great).
I also have to mention it was my first time goofing around with the 5F6 preamp... and although it's a classic among classics, it may just be not quite my cup of tea.
I found the tone to seriously lack some headroom and to be too loose (lack of punch). I also found the EQ controls to be not as effective as on the BF amps.
The speaker I plugged it in was an Eminence Legend Alnico (P12R style). It's a very loose speaker with lotsa breakup and midrange. This plus the cathode boased power section might have contributed to "too tweedy a tone".

However the reverb is working good at this position (minus the hum of course) and I'm sure you can have a great "Tweed reverb" amp this way.
(I'm sure a 4x10", LTP PI and massive iron would have tighten it considerably and made it sound fantastic).

I finally converted this "Tweed Deluxe Bassman Reverb" to something closer to a Princeton Reverb. Dropped the cathode follower, used this triode //ing the first gain stage triode, slipped the 5F6 tone stack between the two first triode. The tone is just fantastic now. Way tighter, and richer too.


As for my hum on the verb, could someone help me?
Spent the whole day trying to eliminate without any success. The chassis is very small (5E3 sized) and I had trouble stuffing the 4 12AX7 down there, so maybe leadress is not optimal.
Here is what I done to trouble shoot it:
- swapped tubes
- replaced the cathode bypass caps in the verb driver and recovery circuit
- pulled out the reverb out jack = still humming
- pulled out the reverb in jack = still humming but mucho less
- poked around the transformers to see if that was transformer hum
- poked around the reverb pot which is close to a big B+ filtering cap.
I had no success still humming.
May you please help me sorting out this problem?

Thanks a bunch,

[tubeswell]

Hi Victor

Great to hear your report of my experimental circuit drawing - so (albeit for the hum and tone problems) you think the reverb sounded plausible that way huh?

Are you able to post a schematic of your end result at all?

Hum has a number of possible sources. Off the top of my head right now, some of them are

Bad wiring - leading to stray induced coupling of unwanted AC from power supply/heater circuit into the signal path. Keep AC pairs twisted together (cancels out the EMR around each wire - since the pairs are at opposite polarity) and keep them away from signal wires as far as possible, and where they have to cross each other, only cross at right-angles. Also if you keep wires as much as possible right next to the chassis, and as short as possible this helps to eat-up any EMR surrounding the wires. I usually shield input signal wires to the grid of the first stage and signal wires to the grid of the second stage - with the cable shield grounded at one end only.

Bad grounding - leading to creation of unwanted small voltage differences in the ground return path(s), which act to induce hum into the signal path. I use a split ground system, where I have power amp and preamp ground return points going to separate ends of the chassis (pre-amp to input socket ground, and power amp to one of the PT bolts), with each grounding point going by its own separate wire. I keep the pre-amp filter cap ground with the pre-amp grounding points. Other do it differently, but that works for me

Bad soldering - dirty joints etc inducing hum - go back and re-flow the joints with a clean iron tip. (Keep the iron tip clean by wiping it on a wet rag/sponge in between each solder)

Bad/overcooked resistors (esp CCs) that got too hot when you were soldering them in - causing values to drift, or causing the resistors to begin to disintegrate - put replacements in taking care not to cook them

[MWJB]

"I also found the EQ controls to be not as effective as on the BF amps." this is typical of the 5F6A type circuit, the tone controls & volume are isolated from the volume control and there is more gain, thus the tone controls have less effect.

4x 12AX7? As Tubeswell says a schem would be good, but given the fact that you are cramming everything into a small chassis, pictuires would be even better.

"The chassis is very small (5E3 sized) and I had trouble stuffing the 4 12AX7 down there, so maybe leadress is not optimal." Aaah, I have doubts as to whether the circuit will fit happily into a chassis this small?

First I'd try a much bigger filter cap at the preamp, if you are only using 10uf, try 40uf see what happens.

[MWJB]

In fact, giving V1 it's own filter cap and another B+ dropping node (resistor value need not be very big, <1K?) might be a good idea.

[Victor Denance]

Thanks for your replies and interest,

Tubeswell, we apparently have the same way of wiring our grounds. And as for the filaments, they are well twisted and I did my best to put them away from the signal wires.

I can't draw a schemo, but basically it's a Princeton Reverb preamp and reverb circuit, with two input gain stages in // and 5F6 tone stack.
I also lowered the 3.3M dry signal reducer resistor to 270k, which limits the usable amount of reverb and increases gain very nicely (I have a small tank which is not good for lotsa verb and surf verb is not my thing anyways).
The PI is the standard cathodyne but fixed biased (Paul C mod).
Power amp is 5E3.
My power supply is much like the Princeton Reverb's (except the PI as its own supply tap). I do have a bit of hum due to lack of filtering (my caps are 40/20/20/20uF) because that's what I had lying around but I'll stuff 40uF throughout asap and I'm sure this will take care of the slight hum.
Since space was problem, I dropped the tube recto and replaced it with a 90ohm 10W resistor to emulate the sag of a GZ34.

It sounds really great, but that reverb humming does indeed suck big time.
Also, another problem which might be related to that verb hum is I always have a bit of reverb, even when knob is turned to 0.
Maybe this hint could help you trace the problem down?

Thanks for reading my literature!

[Victor Denance]

Also, another problem which might be related to that verb hum is I always have a bit of reverb, even when knob is turned to 0.

I also realised the hum was reverberated (realised this while plugging in and out the reverb input jack).
Does it mean the hum source is in the verb driver circuit?
(there's no hum when turning up the volume control, just the reverb control).

[tubeswell]

Reverb will always tend to accentuate any other noises in the amp, because it is the most sensitive circuit in the amp. However having said that, did you try swapping any of the tubes (to see if it was a hummy tube)?

[tubeswell]

5F6A trem'n'verb v6

Okay tonight I came up with the revision in the attachment (based on discussions to-date). The reverb takes its signal before the direct coupled pair and injects it after the tone stack with 270k mixing resistors. The reverb has a separate tone control right after the level control. I also have put the bias vary trem back in after contacting the winder and finding that the heater is rated for 5A.

The bits I particularly wouldn't mind some more feedback/comment on are the red bits in the power supply filter and the green mixing bits prior to the LTP.

1) Do I need the red bits? (I initially was worried about exceeding the GZ34 max reservoir capacity with 50uF, hence I chucked it in, but I think I was wrong about that, and now I am not sure whether the initial small CRC filter stage would make it too saggy). I am quite keen to save some space in the chassis if these filter caps etc are superfluous.

2) In regard to the green bits- will 270k mixing resistors work well here? (I am not sure whether it would lose too much of its tweed bassman-like character this way - hell it is getting away from a tweed bassman after all). Or do I really need to add another driver stage between these mixing resistors and the LTP?

I really am after a tweed bassman-come-JTM45 with reverb and tube-driven bias-vary trem, with 2 x greenbacks in a combo. (I realise this version is goin' to have way too many knobs)

[MWJB]

1) No, you don't need the red bits, the GZ34 will be perfectly happy with 50uf for the first filter stage. One change, connect the B+ filter cap assembly to the hot side of the standby, not the downstream side as you have it now. A single 40-50uf 500v cap at the screen node would suffice, if you really are going to use a PT with a 325-0-325 B+ secondary.

[MWJB]

A simpler circuit idea has just popped to mind...plug your guitar into the preferred channel, take the reverb out cable and plug it into the #1 input jack of the other channel. The tone control could now go on the other channel's volume control 5F2A style, to EQ the wet signal?

Could also be tapped after the tone stack, or before?

It's a compromise that leaves you with one channel but careful choice of the instrument channel vol control's bright cap (if required) will sort that. No additional gain on the dry signal...just thinking out loud...no guarantees, but should work.

[tubeswell]

Thanks for that suggestion MWJB

I think I understood that. (See attachment for what I think you mean't)