Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TweeDLX on October 05, 2009, 10:58:57 am
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I recently installed this PPIMV in my SF Princeton. Seemed to work just fine, with the exception of some hum when turned all the way down. I let a friend borrow it, and when I went to turn it on after he had used it, the lights came on, tubes got hot, but no sound. Getting about 1.4VDC on the plates of the power tubes. Couldn't get any reading for the bias. I also recently installed a new PT (David Allen's TP25). Any ideas? Where to start looking? Thanks!
Mike
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Ouch! That's probably gonna be expensive to fix.
The circuit you posted will kill an amp in short order. Your MV varies the bias to the output tubes and when set for minimum the bias is zero. The output tubes will conduct heavily, probably killing the tubes. The excess current may burn open the OT primary (Probably did, since you only have 1.4vdc plate voltage). This may also kill the PT It's definitely been stressed.
Remove that mod. Then pull the output tubes and check for some negative bias voltage on pins 5 of each output tube socket. Now check for B+ voltage on pins 3 and pins 4 of each socket. Tell us what you have.
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I've made the same mistake eyeball wiring that MV in a circuit. The stacked volume ctl comes after the 220k resistors with another set of coupling caps going to the power tube grids. The way it's wired, you've made the pot part of the bias circuit. It'll red plate the tubes muchO fastO.
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That can be a great master volume when it is installed right.
I don't know if there is any difference on a cathodyne type setup(and why there would be if so...), but here are some better instructions on how to wire that style master volume up.
It goes by a few names: Ken Fisher Type-2 master and Lar-Mar master over on metroamps,
(http://i122.photobucket.com/albums/o243/topbrent/Schematics/KenFisherType2.jpg)
(http://i122.photobucket.com/albums/o243/topbrent/Schematics/LarMarMasterVolumeupdated.jpg)
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Thanks for your replies. I figured it was because of some dumb-ass thing I did. :embarrassed: I'll see what I can find out tonight. Hoping I didn't fry the new PT. I do have a spare OT.
Mike
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Removed the MV, replaced original wiring. Removed power tubes and measured the following voltages:
Pin 3 = .809V, .810V
Pin 4 = .844V, .843V
Pin 5 = 35.19V, 35.22V
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Pin 5 looks OK. I'm assuming you really measured negative 35 volts. Still no plate voltage though. Check for B+ on the OT primary center tap lead (usually red). You can easily get to this point by just probing pin 2 of the rectifier tube. Should be about 420Vdc. If voltage is good at that point, the OT primary is likely open. You can double check the OT primary by checking the resistance between the CT and each plate lead.
If you don't have B+ on pin 2 of the rectifier, check for approx 340 VAC on pins 4 and 6 of the rectifier. If the VAC is good, replace the recto tube. If VAC is bad, you'll most likely need to replace the PT.
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The OT CT measured .598V
Rect. pin 2 was .800V
Rect. pins 4 & 6 were fluctuating too rapidly on my DMM (couldn't read), but approx 250mV
Looks like it may be new PT/OT time. :cry: Thanks for the help Sluckey!
Mike
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I swapped in a new/old PT (I think it's made by NorthLake-from an old Muzak amp), and a Hammond 1645 OT. The thing barely fit back into the cab. I'm now reading about 495 on the plates with a 5R4 rectifier. Seems pretty high, so I swapped out the 5881's for a pair of JJ6L6's I had lying around and biased to 35mA. This "Princeton" sounds awesome. Big, loud, and clean. I had previously removed the trem to use the extra side of the 12AX7 as an extra gain stage and added a mid control. I'm thinking I could try a 5V4, or maybe even a 5U4GB (since the PT will handle it) to drop some voltage. Anyone have any suggestions/warnings? I appreciate all comments.
Mike
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I found this old post and had a question...
Im using the Type 2 as in the photo posted below..
I noticed on my Diaz amps, Cesar never used the (2) 2M2 resistors.. What affect would this have?
Also, the schematic I got from the Ceriatone site is using a 250 K pot, not a 100 K... Again, what would the affect be?
thanks ppl..
todd
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It's my understanding that putting the 2.2M across the 250K pot gives a total resistance of 220K or thereabouts... but what the 2.2M resistor does additionally is give the grid a path to ground should the wiper lift away inside the pot, which is the biggest reason it's used.
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I thought it gave the bias a path to the output tubes in case the wiper lifts? I think it's both?
No bias, tubes burn up and maybe OT too. :w2:
Brad :icon_biggrin:
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So there would be no tonal affect?
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You can't use that master on a cathodyne PI.There are ones to use that will be fine but not the same as a long tailed pair PI.
Answer to the question about the 2.2meg resistors:no they don't alter tone.
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2.2M 2 watt resistors are hard to find..... Any suggestions?
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Why 2 watt?
Your not droping any voltage through them. I would think 1/2w to 1w metal film would be plenty?
Brad :icon_biggrin:
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Cool,, thanks much!!
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This crossed my mind, so I thought I'd ask..
Could the PPIMV be applied both ways?
Marshall 2203....
1) Replace the existing MV with PPIMV ..
Or
2) leave the existing MV & Vol, and install the PPIMV ..
I've done the first, but not sure about the second..
I do like the sounds I'm getting from this beast...