Princeton Reverb Re-work

[frankeg]

Hello All, Just joined this forum and this is my 1st post. I’m working on a friends 67 Princeton Reverb that has had some mods done to it many years ago which removed the vibrato and reverb circuitry, changed the tube rectifier to diodes and added a 12” speaker. He loves his sound but wants the reverb back. The old reverb pot is being used as a master volume and the vibrato speed pot was changed to a 5k pot and wired between the power tubes (6V6’s) common cathode and ground. The original bias circuit is intact going through the vibrato’s intensity pot to the pwr tubes grid but the other side of the pot is disconnected from the vibrato tube side. I measured -34 vdc bias at the pwr tube pins with the tubes removed. The speed pot does not change the bias voltage but I suspect it does change the cathode current which I couldn't check at the time. It does change the sound. It has been like this for 25 years and the owner likes it but I am skeptical. It seems like a combination cathode and fixed bias? Can you shed some light on this arrangement? Should I leave it alone? The owner doesn’t want the vibrato but does want a master vol. I thinking I can use the intensity pot’s slot by removing the bias connection and wiring it directly to the grid resistors. I will need a new 1meg pot which I’ll connect either before or after the reverb return. Any advice on what I’m doing would be appreciated. I’d also like any advice on what reverb tank to use. I am a electronic technician (Nuclear Power) and have built two tube amps as a hobby. I learned tubes in the Army (Signal Corps) and have read many tube amp books. I generally know what I’m doing but I certainly don’t know it all. I am a bit nervous about working on someone else’s vintage tube amp so I want to be certain before I start. Thank you in advance for any advice you can offer. frankeg

[tubeswell]

Hi Frank and welcome to the forum

If its been modded like that, it definitely ain't 'vintage' anymore (so I wouldn't fret too much about re-modding it). You could either just leave the bias circuit etc alone, and simply reinstate the reverb circuit; or

What would be interesting is to stick it back the way it originally was intended and see how your friend likes the sound of that after 25 years. 2CW

[frankeg]

Thanks for the welcome. I'm with you tubeswell ! With the exception of the 12" Celestion he added, I'd take it back to stock! But I'll honor his request. It's funny too because the master vol he likes doesn't do a whole lot. I have a "English" flavor amp I built with a long-tail-pair PI and master vol that really smokes with the pre's dimed and the master on low (although I like pwr tube distortion better). I think the ear/hearing changes the more they're exposed to different styles of tube amp. I thought I was an el-84 kind of guy but I do like the sound of his 6V6's through that vintage 30 speaker. Have you seen or heard about this particular bias circuit I explained in my 1st post? I'd still like to get some opinions on it....thanks, frankeg

[tubeswell]

Hmmm... the 5k pot wired that way would appear at first glance to simply be an adjustable cathode-bias control. Is it wired in parallel with the cathode resistor, with another resistor in series? (5k would be way too much in series on its own - most that you would want for a cathode resistor in one of those amps is between 250R to 330R - that's why I'm curious as to how its wired. If it was in parallel with say 330R, well that's different, as long as you have a about another 5k in series with the ground side of the pot, then you'd get a bit of range with which to adjust the bias in the ballpark)

[frankeg]

The 5k pot is wired between the cathode and ground directly. There is nothing else in parallel or series with it. The original design has the cathodes tied together and going straight to ground. The -34 vdc bias supply is connected to the power tube grids. As you say, 5k would be way too large for a straight cathode biased power section however this arrangement keeps the -34 vdc bias supply intact while adding the cathode pot. The sound range is from very bright with pot at zero to warm/brown when the pot is increased. I really do need to test it with current to see how much is changes it. I was at 1st worried that the cathode current could/should burn out the pot but it hasn't in all these years and after thinking about it, we're talking milli-amps here for cathode current right?

[tubeswell]

Ah so its a combination fixed bias and variable cathode bias then? And yes there are only a few mA on the cathodes, probably something like 80mA altogether for both 6V6s. I'm curious what it measures with the pot dimed. (Are you positive its 5k, and not something like 500R? or 50R? - did you actually measure the pot resistance? I imagine that with 5k the bias would be pretty cold.)

[frankeg]

For one, the pot is not stock but rather a mini pot which appears heavey duty. I first traced both wires - one went directly to chassis ground and the other went to the 6V6's shared cathodes. I measured from the cathode to ground with the tubes removed and got the 5k ohms. I did not disconnect the pot and measure across it. When I get it back I will. I'm thinking that because it still has a negative bias supply on the grids that this pot doesn't "act" the same as it would if it was strickly a cathode biased circuit. I agree with you that any tweek above zero on this pot should in "effect" add a more negative bias to the tubes and the sound I got while adjusting it concurs. More pot resistance - more tube grit. The "sound" range is narrow, as I mentioned earlier, from somwhat bright (brittle) to warmer/browner and not much distortion. This is a very interesting idea/mod to me and I thought surely it has been used before but perhaps I'm wrong and it really is unique. This was done in the early 80's and the owner said that the "kid" was a tech school grad who was into building his own amps. He fiddled with this one for some time before he was satisfied. frankeg

[JoeM]

With the 5K pot 'dimed', i.e. set to the max 5k rotation, what does the voltage at the cathodes measure?

A stock PR, or with a few real basic mods is a great sounding amp. I would seriously consider returning the amp to stock. (With the mods that you can find here in the forum).

[frankeg]

Hi Joe, I did not measure the voltage across the pot which in this case is the cathodes. In hind sight, I should have. I'd expect to see a voltage drop of course and I cound then figure the current draw but alas I missed it. I was just inspecting this amp prior to re-capping and restoring the reverb circuit to get an estimate on what was needed. I had the amp over the Christmas holiday and returned it the following monday. I will check that voltage as well as the cathode current when I get the amp back. I was just wondering if any of the more experienced folks around here have seen this arrangement before.

[JoeM]

I've never seen that arrangement. (And I dont see how it could sound good.) An amp builder friend once used a 100 ohm pot in series with a fixed resistor in a cathode biased amp, but once set where he liked it, it wasnt changed, might as well have just used one resistor.

[frankeg]

I agree Joe. It has a very limited effect in what sounds good. I appreciate everyones comments on this. We agree on what should be done so I'll approach the owner with it and see what he thinks. You know I could just convert the amp to cathode bias and place a smaller r-value pot in series with a fixed cathode resistor and he'd never know the difference.... but that wouldn't be right... He seems to want something to tweek... You know how those guitar players are...frankeg

[frankeg]

The thought has occurred to me that I could insert a small value pot in series with the bias voltage. If I do it correctly, I could allow a few volts change up or down to give him something to tweek. Also, when you say "back to stock", do you include the rectifier? I did notice another curious thing...The stock cap can has four 20mfd supply filters but his had a four 40mfd can. If I'm thinking correctly, isn't this (along with the diode rectifier change) increasing all of the amps dc voltages? Perhaps that is why the cathode mod was added.

[tubeswell]

In my experience/experiments, a 40uF reservoir won't give you much more B+ than 20uF all other things being equal (i.e.; with the same rectifier and PT and tube load). It'll raise the B+ by 8-10 V at the most. And the other caps in the can at 40uF might just stiffen the sound a tiny bit. Sixpence each way - if the can is working, I'd just leave it. On the other hand, if its more than about 10 years old, why not get another one? Weber makes cans for PR clones

As for the bias adjustment pot, that's pretty much standard on most amps these days. But I'd have it wired as a variable resistor on the leg of the voltage divider in series with another resistor. Be sure to wire the wiper to one of the other pot lugs, so that if the wiper contact fails, the bias circuit won't go open. 2CW

[ThermionicScott]

I've never seen that arrangement. (And I dont see how it could sound good.) An amp builder friend once used a 100 ohm pot in series with a fixed resistor in a cathode biased amp, but once set where he liked it, it wasnt changed, might as well have just used one resistor.

Tangentially, a 100-ohm pot with each extreme connected to a power tube cathode, and the wiper to ground, is a good way to adjust for unbalanced tubes. And it has a nicer failure mode (no current flows) than a balance adjust in the bias supply.

- Scott

[frankeg]

Thanks tubeswell thats good advice. Happy New Year to ya! I'm going to try to talk my friend into going back to a tube rectifier. I could get one of those weber diode rectifier cans which fit in the tube socket if he really dislikes the tube. He plays classic rock, blues and country so I can't imagine that he won't like the tube rectifier. As far as the cap can goes, it is over 20 years old and the cathode electrolytics are still stock. I planned all along to re-cap it. I'm thinking I'll stick with the 40 mdf's. I do like the bias circuit pot! As for the master volume, I can use the vibrato intensity pot for that and wire the bias circuit directly to the grid resistors. He doesn't want the vibrato. I'm not sure yet where I should wire in the master vol - either before or after the reverb return, any thoughts on that? Thanks again for the advice. I really appreciate it.....I do a lot of thinking and seeking before I heat up the sodering iron. That has generally kept me out of trouble over the years. It's really nice to have a forum like this to share ideas... By the way, whats your favorite amp?