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Hammond Deluxe Conversion Amp

Structo

Member
Messages
9,556
I recently picked up a Hammond organ amp for a project.
Originally I had planned to make use of everything I could with the parts in it, but after getting into it I decide to gut it and just use the power transformer and rectifier socket.
The good news is, was that it came with some vintage tubes, two Sylvania 6V6GT's, two 12AU7's and one 12AX7.

Anyway, I found a schematic on the web that approximated what I wanted.
I did not want the dual inputs of the Tweed Deluxe because you waste half of a tube that is not used.

Here is the schematic I used.


The problem I am having now that I have the amp running is it has way too much gain. The only way I can calm it down is to put a 12AU7 in V1 or V2 which is the phase inverter.
If I use a 12AX7 in both V1 & V2, it will oscillate when the tone control is dimed.
I added a Deluxe style tone control.
I also have installed a global feedback with a presence pot as well.

I've experimented with different plate load and cathode resistors and still can't seem to tame it.

One issue that bugs me is with the 1 Meg volume pot, It's range of sweep is very small.
In other words, when I turn it up, it goes from clean to mean within a fraction of a turn.
I've experimented with the 470K grid leak resistor shown on the plan, trying 100K, 50K, and 10K with no noticeable difference.

So how can I lower the gain in V1 (I really want to be able to use a 12AX7 there)?

I have also played around with values in the PI to reduce gain there as well.
But the volume pot sweep range is probably bugging me the most right now.

The amp.


Lead dress isn't the best due to cramped quarters but the noise floor is low and it only oscillates if the tone pot and volume are dimed.
I put a 47pf cap across the PI plates to help this.
 

TweeDLX

Member
Messages
3,756
I'm far from an expert, but have you tried increasing the value of the input grid resistor? I reduced the values of the CK bypass caps and coupling caps in my 5E3 clone to increase clean headroom with good results. I know this changes the character of the amp, but it also seemed to give me a little more control over the volume sweep.

Mike
 

Structo

Member
Messages
9,556
As far as I know the input resistor is there to stop RF interference and does little to affect the gain, to a point.

The cathode bypass caps have a bigger influence on frequencies.
But a bypassed cathode does increase gain so I have experimented with no bypass caps on v1b and that did help a little.

I just can't figure out why the volume pot is so touchy.
It's like it's on or off with very little sweep.
I suppose it could be a faulty pot and since I don't have any other 1Meg pots around I can't substitute that out for now.
 

SatelliteAmps

Member
Messages
6,182
Just because it is a 1Meg pot doesn't mean it's the right pot. Different styles of pots have different sweeps. You might want to try a linear pot instead of an audio pot.

There are other ways to reduce gain, but I would get the amp running correct first, then start tweaking. The 5E3 Deluxe is a pretty gnarly little amp, so it also might be running just fine. Hard to say off what you explained. Have you played another Deluxe to see how they compare?
 

Structo

Member
Messages
9,556
No, that is one of my problems.
I haven't played a similar amp.

This amp is a bit of a bastardized Deluxe since I am using both halves of the 12ax7 for gain, with one input.
I've been plugging in a lot of different values for the gain stages with little improvement.
Last night I changed the plate and cathode resistors again, but I'll have to wait until later today to try it out.
 




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