Hammond reverb amp for guitar

[ubxf]

Hello,
i converted this amp for guitar and it works well with very few changes. But i was wondering if i could do a few tweaks. Currently i only changed the grid stopper and the grid leak resistors and replaced R6 by a pot so i have a volume control. I was wondering if adding a bypass cap in parallel to R2 or changing the value of R9 would be worthwhile changes. I could try it i guess but i don't know anything about tube designs and don't want to ruin a good amp by trying things at random.

 

[abbey road d enfer]

What would be really useful is to increase considerably R1a. As it is, it chokes the gutar pick-ups. 1 Meg is the usual value there.
You could also reduce the value (or even disconnect) C5. It shaves a little too much treble IMO.

 

[ubxf]

R1 and R1a are already changed to 33k and 1 Meg. i'll  experiment with C5 and listen for its effect on treble thanks.
Is the feedback loop from the speaker necessary?

 

[abbey road d enfer]

R1 and R1a are already changed to 33k and 1 Meg. i'll  experiment with C5 and listen for its effect on treble thanks.
Is the feedback loop from the speaker necessary?

It's debatable.
Feedback reduces THD, until the moment open-loop gain starts to run off, so the transition from clean to dirty is not gradual. It also extends the BW, which is appreciated for an organ, not so necessary for a gtr amp.
Many guitar amps do without global NFB, for a touch of 2nd harmonic.

 

[ubxf]

Thank you, i'll try without and listen. Taking out C5 was great.  At small gain it was a very subtle difference but wide open is very noticeable .

 

[Whoops]

Hi,
I advise you also to post in this forum:

https://el34world.com/Forum/index.php

It's dedicated to tube guitar amplifiers a lot of knowledgeable and helpful people there also,
they will give for sure some good advise for that circuit

 

[ubxf]

Thanks for the recommendation, i might go there but what i've seen there so far is people striping the amp to the bare chassis and transformer and rebuilding something else.
While i'm sure they get very good results, i'm trying to keep this amp mostly unmolested.
So far i like what i'm hearing. I'm not sure i even want to add a tone stack. A change of value on a component or removing one (one at a time) is interesting to me so i can learn both the electronic and what to listen for.

 

[Whoops]

i might go there but what i've seen there so far is people striping the amp

that's strange...
I would say you haven't seen enough

 

[ubxf]

Sure, i hope i'm wrong.

 

[Michael Tibes]

As inspiration: in my experience amps benefit from eq voicing for guitar use, they don't sound good to me with a flat response. One of my favourite amps is an old Dynacord power amp which was quite similar to your amp (same tubes and basic circuit). I modded it with a typical tonestack circuit. For inspiration I looked at some amp schematics with similar tubes. Gutting the amp and rebuilding it from scratch might be easier / cleaner / faster and has the benefit of having the reliability of new components, reusing the old components might lead to more 'character'. I tried to reuse as much as possible. That way I got myself a modern amp with an old soul. It was a big effort, but in hindsight it was totally worth it, I got myself a unique amp which has been used a lot.

Michael

 

[ubxf]

Thanks Michael. I totally understand and i agree with you. But in this case i don't know if it's luck but i've got something that doesn't sound half bad after changing just a couple of components. Adjusting the volume and tone on the guitar does a lot already .
I'm finally realizing after 20 years what change one thing at a time and listen means.
The problem is i still don't know what to change. I replaced C2 and C4 with the same value but better capacitors and definitely heard the change.

 

[ubxf]

I'm trying to understand R3 R4 andR5. Is R5 and C3 an extension of the power supply?
What is the reason for R3 ? and C2 ?

 

[abbey road d enfer]

I'm trying to understand R3 R4 andR5. Is R5 and C3 an extension of the power supply?
What is the reason for R3 ? and C2 ?

In order to optimize noise performance, the 1st stage runs at 0.3mA plate current. In typical cathode bias, the cathode resistor would need to be about 2.2kohm, but then there would not be enough gain.
The designer chose this method of injecting current in the cathode resistor via R3, which is as valid as any other. I would probably not have done that, but each one his own...
C2 is there to block RF.

 

[fazer]

Dr Z built an amp called the Karman Gia.  It was based off thE AO35.  It used a one knob tone unique in design based on one used in a Selmer amp.  That amp is one of my favorite sounding studio amps.  Dean parks has a YouTube video playing that amp.  Parks is a premier studio guitarist and could make anything sound good.  The magic  in his opinion is the tone control Because it works by rolling off the bass.  If you play by yourself, you tend to like more bass for a fatter tone but when playing with a bass player it can sound boomy And conflicting so rolling off the bottom allows the guitar to leave room for the bass guitar in the low end.  The amp would be more versatile with an eq IMO.  That said it’s what ever fires your rocket.

  You can find a schematic for this amp I believe called the Phoenix and also a layout.  I do admire your dedication to understanding the components influence on the circuit.

 

[tommia]

I just googled the schematic. If you use some toggle switches you can dial in the eq, different cathode circuits for V1a, different value for lets say R9. Now you can hear what every changement does.
Keep us informed!

 

[abbey road d enfer]

The Dean Parks explanation seemed a tad dubious, so I simulated the tone circuit and admittedly, at full CW, LF are cut-off significantly, with a 6dB/octave slope resulting in 100Hz being ca. 30dB below 3k.
Conversely, at full CCW, the response is that of a RC low-pass, with 100Hz being ca. 24 dB above 3k.
In-between there are various low-Q notches varying from -23dB @900Hz to -28dB @ 2kHz.
Midrange notch is almost mandatory for electric guitar. The typical Fender/Marshall stack does that, but generally at a lower frequencu (typically between 400Hz-1kHz).