Organ amp to guitar amp. Power supply question

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A0—48 amp from an Everett organ. Bit unusual with pentode starting the pre amp but that's Vox Ac15 like I think..

Anyway.. Just starting to get my head around the schematic before deciding what to strip away..

I have a circuit off the HT center tap on the transformer that generates negative 12v & 16v for organ tone circuitry (now scrap)

Really expected to see center tap grounded.. And now the redundant negative 12 & 16 will have zero load I'm unsure how the secondary will perform.

So

A) can I ground the center tap and dump the negative voltage components in the trash.
B) do I need to replace the circuitry with a limiting load of some description.
Page 20 in the manual below I think

https://archive.org/download/Hammon.../Hammond2000SeriesChordOrganServiceManual.pdf
 
Fantastic news.
Even with instructions in full colour.
Within one hour.
In the middle of the night, (though you could be in a different time zone)

My soldering iron takes almost that long to warm up..

I need to find an Ac15 schematic I can actually read... and decide what I might try to follow

Dig out the variac and persuade my wife to treat me to a new multimeter for Christmas.
 
Replace R357 with a wire link. Job done.


Here is a copy of a diagramme I found some time ago. I am not aware of any copyright as it is not an original.
 

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AC15 schematic is so so nice too.
I've never owned, seen, or even heard one in the flesh. But they do (did?) have a reputation for suffering from microphony.

I've read that VOX lifted the very high voltage gain pentode circuit straight from one of the valve manufacturer's catalogs. It worked very well as a microphone preamp, or at the input stage of a record player.

But electric guitars spit out a much bigger signal, and tend to be played very much louder than record players. Now the tail wagged the dog (speaker vibrates the EF86) instead of the other way around, and with all that voltage gain in one stage, this would generate enough signal to feed back to the loudspeaker and keep the unpleasant howling going.

In the last ten or fifteen years, a few "boutique" American guitar amp manufacturers have made the same mistake, usually borrowing the same ancient EF86 circuit and using it unchanged. And the results are usually the same - within short order, microphony is a major problem, and the EF86 needs to be replaced yet again. And again. And again.

My suggestion, therefore, is to use a half-12AX7 as the input stage. If you want to use the EF86, re-design the stage to drop the voltage gain considerably (this is most easily done by drastically dropping the B+ voltage to the pentode), and use it as the second or third gain stage in the preamp. This reduces the amount of gain after the pentode, and also strengthens the signal level at the pentode input, both of which reduce the tendency to microphony.

Merlin Blencowe has a helpful chapter on small-signal pentode design for guitar amps: The Valve Wizard -Small Signal Pentode

My personal preference is not only to avoid a pentode at the input stage, but also to try to design small-signal pentode stages to have about the same voltage gain and output impedance as a typical half-12AX7 triode gain stage.

This means using something close to 40k as the pentode's anode resistor to get the output impedance right, and then lowering the B+ to typically around 150 volts to get the voltage gain right (say x50 or x60). Maybe even lower, depending on the gm of the particular pentode you happen to be using.

Keep in mind voltage gain for a pentode is gm x Ra, so once you decide Ra is going to be 39k and gain is going to be 60x, you know the pentode has to be at an operating point where its transconductance is about 1500 uA/V or mmho or uS, which ever is your unit of choice.

That still leaves you with the job of working out the screen voltage and cathode resistor to set the bias where you want it. Vgk usually sets the input overload point, and with that decided, all that's left is to work out the screen grid voltage and appropriate biasing resistor and bypass cap. :)


-Gnobuddy
 
Designing Tube Preamps for guitar and Bass second edition has become my evening reading of choice..
You picked a good one. I've worn out my copy from sheer use. It's been with me in airport lounges, doctor's offices, on vacation, and in my hand while I ate my sandwich at countless solitary lunches.

I used to own several other books about valve guitar amps. I didn't bother bringing them with me when I moved from the USA to Canada.

Thank you, Mr. Blencowe!

-Gnobuddy
 
Out of the afore mentioned book came a reference to this amp Matchless Clubman

http://www.prowessamplifiers.com/schematics/post/matchless_clubman.pdf

12ax7... A small pentode... Second 12ax7 inversion... Push pull output stage..

Damn me if this doesn't look just like my Organ amp with a repositioned pentode and half a12ax7 stage robbed back from the tremolo circuit.

Could I splice the reverb circuit and tank into the matchless efx send and return..

Cirtainly looks like my most promising find yet..

Any ideas folk?
 

PRR

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> Damn me if this doesn't look just like my Organ amp

ALL Audio Amplifiers are assembled from a small collection of basic building blocks.

Differences in gain structure and tone shaping.

The organ amp is already "an instrument amp" and thus kin to guitar amps (even if Everett doesn't like it).
 
....Matchless Clubman....
Perhaps you've already listened to these? If not, they might give you some idea whether or not you would enjoy a DIY version of it:

1) YouTube

2) YouTube

...just like my Organ amp with a repositioned pentode and half a12ax7 stage robbed back from the tremolo circuit.
That's kinda what I was thinking earlier. Your well-endowed donor chassis puts you in the happy position of starting with a circuit that has an ample number of triodes, a small signal pentode, and a pair of output pentodes. That gives you a lot of possibilities to play with and enjoy!

Could I splice the reverb circuit and tank into the matchless efx send and return..
That certainly seems like one option. Another would be to remove C15 and inject the reverb signal (after make-up gain) into the unused grid in the long-tailed pair - pin 7, V4-2 in your schematic.

Incidentally, I am deeply suspicious of the indicated 10k value of R15 (first anode resistor). Perhaps it is actually 100k.

Any ideas folk?
I will confess that I was stunned into horrified silence by the "design" of the FX send.

Apparently the designer decided to start with an already extremely high 220k output impedance from a small-signal pentode, run that through an even higher impedance 1M pot (which can add more than 250k additional source impedance), and for good measure, run the signal through yet another 150k resistor before hitting the FX "Send" jack.

Even with the 470k (R6) in place, it looks like you could end up with a "send" signal source impedance as high as 270 kilo ohms. Anything higher than, say, 10k is in "Tut-tut, that's shameful!" territory.

Another horrifying feature is that, since the small-signal pentode is run on a very high 410V B+, and pentodes can usually spit out a signal that's only a few tens of volts less than th B+, we can expect up to maybe 380 - 390 volts peak to peak at the 6HS7 anode when it is being overdriven. If the 1M "Send Level" pot is now turned up to full, the entire 380 Vpp is fed to R4/R6, which attenuates it to a still terrifying 290 volts peak-to-peak before sending it to the "FX Send" jack. :eek: A sane design would never spit out more than perhaps 3 volts peak-to-peak at the FX Send.

Of course, you could always adopt the overall topology (triode -> pentode -> PI -> output), while fixing the badly designed bits of the Clubman's design.

I can't understand why the input triodes would be run from a 205V B+, and the small-signal pentode run from 410 V B+. To me it would make more sense to do the opposite - these little pentodes run very happily on much lower voltages than a half-12AX7 will, and using a low B+ is the easy way to avoid having too much gain in that one stage. But, if the Clubman sounds good to you, then that's all that matters. The scary and inept FX send can be fixed without changing any other part of the amp, so the original sound can be preserved.


-Gnobuddy
 
It's worthwhile trying to connect the power transformer secondary CT directly to the C329 negative terminal (which is then grounded either by deleting R357 and linking to bottom of C330 can, or shorting out R357 as pointed out earlier) - as that should avoid any hum creeping in from that rectifier stage.

It's often not too difficult to unbolt the input (EF86) valve socket and slip in some neoprene style mechanical compliance between the chassis and socket. That can attenuate any vibrations making it to the valve, and similarly suppress noticeable microphonic sounds. If you have a stash of EF86, then tube rolling may identify a tube with noticeably less microphonic sound output. Just even sitting the amp chassis on a compliant rubber feet can help. Those techniques may be a lot easier than redesigning the electronics.
 
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