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PRNDL 4th December 2007 06:47 PM

Precision PA conversion help needed
2 Attachment(s)
I'm in the process of converting a Precision Model 535 PA amp for use with jazz guitar. It uses a pair of 7868's, 2 - 6EU7 and 1 6C4 preamp tubes and a solid state rectifier. Power supply voltages are 437, 410 and 342 volts, although the 6EU7's see about 150.

The sound with guitar is muffled and without enough volume you'd expect from 7868's, which are comparable to 6L6's. Distortion occurs with the preamp or master volume at 6.

The 6EU7's show bad emission quality on my Sencore, but it's showing that a lot lately. The 7868's are new.

One issue is tube price. The 7868's are $25 each on sale, and the 6EU7's are $18 each.

One option would be to redo the amp with 12AX7s and 6L6s into something like a Fender Pro. Before that, I'd like to try a few simple changes and see how it sounds.

I've put together a schematic, which isn't accurate or complete.

I have two main questions.

First, the master volume connects to both the grid of V2a and the James tone stack. This looks like a wiring mistake to me.

Second, I'm considering taking out the feedback, which would entail grounding R20 and disconnecting the wires to R28 and R13.

Does this sound like a good idea?


PRNDL 4th December 2007 08:20 PM

updated schematics
I've updated the schematics.

The 7868's have grounded cathodes and a fixed bias to the grids.

Also, C17 and 18 were connected to a "remote control" jack.
The caps are green and were marked TRW AU-113.

One option may be disconnecting R29/C16, which appears to be a low pass filter (10K and 250p).

Here are some tube voltages.
V1 plate = 60 and 66
V2a plate = 174
V2b plate = 160
V6 plate = 231, cathode = 113
V3 plate = 437, grid = -17, screen = 410

V1 plates are really low. Perhaps a mod would be to remove the C17/18 and bypass R3/10, which would put a 220K plate resistance and raise the plate voltage to 170 or so. A cathode resistor and cap might be nice too.

Thanks again!!

Wavebourn 4th December 2007 10:06 PM

One more error: R20 and R21 has wrong values marked.
V1 should have such low plate voltages, it is a mic preamp.

You should consider of redesigning the whole thing to get a "guitar" sound. Transformers would be Ok, but the amp is not traditional for guitar overdrive. Here was a great thread recently about a guitar amp design, I suggest to read it carefully.

Eli Duttman 5th December 2007 12:24 AM


I remind you that 7868s are as easy to drive as EL84s. No way are you going to get the gain you need for 6L6 "finals".

AAMOF, changing the voltage gain front end into an EF86 might be necessary to give you the overdrive "pickers" seek.

Using a 12AX7 to replace a 6EU7 is (IMO) a "no brainer".

If you come up short for tone stack voltage followers, ZVN0545A MOSFETs work fine DC coupled to 12AX7 anodes.

PRNDL 5th December 2007 01:41 AM

2 Attachment(s)
Yes, I swapped the values for the resistors.
I couldn't find the thread you mentioned -- any idea what to search for?


So far, I've done one thing at a time and still have guitar signal, although the volume and tone hasn't changed much.

The first preamp stage now has 220K plate resistors, which is more appropriate for guitar.

I removed the wire connecting the master volume to the grid and the tone stack. It didn't seem to change much.

There's a really interesting multi-section cap that seems made specifically for the James. It's a thin ceramic square with the numbers 1347043 and PC1964. There are 7 pins, three of which go to the bass pot. Pin 1 goes to resistor R9 and a 100 pF cap to the middle of the treble pot. (My schematic shows a traditional James stack with values for guitar, which is what I'll have to replace this with.)

The main problem is the low volume (my Champ is louder) and the intense scratching. Those 6EU7's might be bad, which means I'll switch to 12AX7's.

The next change is going to be the feedback. I'll probably remove it entirely, based on the 5E5 Pro schematic, although I think some feedback might be appropriate, but not two.

>> 7868s are as easy to drive as EL84s.

Is there a suitable guitar amp (and schematic) you can recommend?

Eli Duttman 5th December 2007 01:48 AM


Is there a suitable guitar amp (and schematic) you can recommend?

Look for something that uses the 7591. The 7868 and 7591 are electrically equivalent.

PRNDL 5th December 2007 02:27 AM

Thanks Eli,

I found the Ampeg Portaflex SB-12, which uses 7868's and 12AX7's with a schematic that is really close.

Wavebourn 5th December 2007 02:41 AM


Originally posted by PRNDL
Yes, I swapped the values for the resistors.
I couldn't find the thread you mentioned -- any idea what to search for?
Here it is:

Also, probably you swapped cathodes, I don't believe that a global negative feedback signal would go to a stage before a timbre controls, most probably it goes to the following stage.

Also, you may try to use a remote cut - off pentode in the stage before a timbre stack such as 6BA6 with a cathode follower after it, you may like it's sound!

rickl 5th December 2007 04:12 AM

I believe a Matchless Spitfire is a 12ax7 EL84 tube amp.

Maybe this will work for you.


PRNDL 6th December 2007 05:39 PM

2 Attachment(s)
OK, I changed the preamp to look more like a Matchless Spitfire with two extra gain stage, so it should have plenty of gain.

Unfortunately, it's not loud at all (about 0.5 watts by my guess).
The sound is thin, which may be due to the tone stack.
It has nasty distortion when you turn the preamp or master volume up past about 6.
Low E notes on the guitar distort like crazy, which may be due to the .047 coupling caps letting through too much bass.

I put in a new JJ 40/20/20 cap can, which didn't change anything. I figured it was due.

Any ideas??

I've attached a schematic with measured voltages for all the tubes.

I'm wondering if the distortion is coming from the cathodyne phase inverter.

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