single channel DR w/o trem

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BushDoctor
Junior tube assistant
Posts: 10
(2/3/04 9:48 pm)
 single channel DR w/o trem
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 I while back I posted about a Princeton R that I converted to a single channel DR w/o trem. I had problems w/ hiss, hum etc. I'm still tinkering but when when I lower the gain w/ an AT7 it quieted down right away. With the AX7 my plate voltages were over 200VDC. I'm still wondering about the effect of omitting the normal channel. Weber and others say get more gain by removing normal chan. tube. I have done this on several of my other amps and they seem noisier. I also tried disconnecting the trem ground leak ,on my other amps, with a pot switch but again the noise and hum are louder. Could there be a method to Leo's madness? It seems the more I tinker the more I get back to the original design. I digress, I'm not a theory guy just tinker and try to understand. If someone could expand or expound on the ommission, and it's effects, of the normal channel on a two channel Fender I might learn a bit more as I already have from this forum,
thanks
 
PhatTele
Junior tube assistant
Posts: 5
(2/4/04 4:28 pm)
 Re: single channel DR w/o trem
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 Does it have anything to do with the fact that V1 and V2 share a common bias off of pin 8 on both tubes?

I'm going to conceptualize here because I'd like to build a single channel AB763 circuit myself...

I would think that if you removed the tube from V1 then the 820 ohm resister would be controlling the bias on pin 8 for V2. However, if you look at the schematic and layout, 1500 (1.5K) seems to be a more standard bias setting for the other sides of those tubes. So, at 820 with V1 removed, V2 is running hotter than it would with a 1500 ohm setting (basically double of the 820).

If that's the case, then that means we need to change the V2 pin 8 bias resister value to something like 1500 ohms in order to be more true to the tone as if V1 were still plugged in.

Did I get that right?
 
BushDoctor
Junior tube assistant
Posts: 11
(2/4/04 7:47 pm)
 reply
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 You are correct, sir. Thanks for the lesson. I replaced the bias R with the 1.5K -major improvement. I can put the 12AX back though the plate voltages are still a little high, 212VDC. Even with a 5U recti I'm at 425VDC B+. A 5AR4 is about 450. I have about 308VDC off the preamp power supply. I think the xfrmr (41316) that Hoffman sells is more suited to Silver-faced DRs. It's quite the hunk of iron with some healthy voltages. I think I could drive 6L6s, maybe boost the OT to one of those 35W tremolux, pro types.
I'm still open to ckt suggstions that this xfrmr might drive -one channel using 3 or 4 preamp tubes. The testbed is this hacked up Princeton Reverb chassis.
 
BushDoctor
Junior tube assistant
Posts: 12
(2/12/04 8:41 pm)
 dropping resistors
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 My amp is coming along nicely. I still have a concern about the Preamp plate voltages on V1 and V2 reverb, 212VDC (remember this is a converted Princeton R, one channel, no trem). Anyway, the Hoffman AB763 design and SFDRs show the dropping resistors at an approx 5:1 ratio, either 4.7K and 1K or 2.2K and 10K on the Fender SFDR schems.. The old BFDRs have 10K and 10K. I think the high plate voltage is still making the amp a little on the noisy side. I've heard some DR folks say they like the SFDR resistor scheme better.
Will this lower the preamp plate voltages?

thanks,
 
GroundhogKen
Forum Moderator
Posts: 2261
(2/12/04 8:47 pm)
 Re: dropping resistors
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 10K/10K will give the lowest preamp voltages.

I don't know if your noise is related to the preamp voltages. Is it hiss, hum, snap, crackle pop?

Ken
 
BushDoctor
Junior tube assistant
Posts: 13
(2/12/04 9:37 pm)
 dropping resistors
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 Mostly hiss. I think the hum is acceptable; drops to almost nothing with V1 pulled. Why such high preamp plate voltages when ther measured V everywhere else seems close to the fender schematic specs? I have a 20, 20, 16,16,16 filter cap config; only boosted the main power supply value by a bit; the original DR design calls for 5-16's.
If 212VDC is OK then I'll move on.
I have swapped in different 12AX's. Currently using a 12AX7-EH in V1that I got from Hoffman.

thanks
 
 
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