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|9/24/1999 10:05 AM|
||RE: voodoo , TIM C ? Vintage Club mods|
Tim C , you mentioned here a sometime ago , you have done some mods to a VC series Crate , removing the 750pf cap across the volume pot and changed one of the coupling caps
on the overdrive channel from .047 to a .0047 .
I have a couple of questions about that, if you don't mind ?
1. I have a VC5115 and there is only a 221pf cap across the volume pot on the CLEAN channel , not the 750pf ? , did you meant that one ? What was the effect , brigthness/brittle disappeared , or what ? On the overdrive channel there's no cap across the "gain" or the "level" pot ?
2. Actually ,which one was the .047 cap on the overdrive channel ,you've changed , first after V1 ,or V2 ? , or what ?
and really to a .0047 (4.7nF) ? What was the result ?
IMO the overdrive channel is the one which needs some tweaking , its harsh , farty (I use lot of a bass ,though) , over distorted (of course its an overdrive channel ,stupid me), but it has an EQ more versatile than on the clean channel, and againts that , have you any suggestions to get it "cleaner" without losing the gain too much ?
Any info appreciated
|9/25/1999 1:44 AM|
The Crate "Vintage Club" amp I modified had 750pf caps across both the normal volume pot and the overdrive volume pot. I removed both of those. The owner brought the amp in to be modified because he thought it was harsh and brittle in both channels. He also did not like the overdrive toneas he thought it to be muddy.
In answer to your questions I offer the following:
1) I would unsolder one end of the 220 pf cap on the normal
volume pot and try the amp to see if you like that sound. It will make the normal channel less harsh. If it semms to lack highs with the 220 pf cap removed, try replacing the 220 pf cap with a 47pf instead. I think you will like that!
2) The .047 cap to remove is the V2 coupling cap. Replace it with a .0047 cap. That will increase the overall gain a bit and change the midrange structure of the amp.
3) Check the tone stack for the overdrive channel and let me know what values yours has. If memory serves me well you should have the traditional .1,.047,250pf tone stack with a 100k slope resistor. If that is the case, Change the .1 and .047 to .015mf@630v respectively and the 250pf cap to 390 pf. Change the slope resistor from 100k to a 47k 1/2 watt.
4) If you amp has 25mf@25v cathode bypass caps on pins 3 and 8 of V2, remove them and replace them with .82mf@630v. caps(available from Angela Instruments).
Let me know if I answered all your questions.
|Book Of The Day||
The Ultimate Tone, Volume III by Kevin O'Connor
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|9/27/1999 10:39 AM|
Do you recall the VCXXXX model number of the Amp you modified ?
I'll try your suggestions , but this has a tone network with 470pf ,.022 ,.022 and 33K , not much different values than what you are suggesting , don't you think
Also , there is only a .68mF tantalum on pin 3 of the V2 to the ground , no bypass cap on pin 8 , just a cathode resistor.
I wish , I'd have a schem , I'm working on it , with the repair guy of the local dealer.
|9/27/1999 4:19 PM|
Don't bypass pin V2 pin 8 if it is a cathode follower, Jukka.
You'll send all the audio signal to ground plus there is a high DC potenial there equal to about half that tube's B+ supply voltage.
|9/28/1999 12:48 AM|
It sounds as if your amp is different than the one I worked on. I don't recall the model number but it utilyzed four EL84 output tubes. I can call the person I did the work for and ascertain the model number if needed.
The tone network values that you said your amp uses are so close to the ones I mentioned that the only change you might consider would be changing the 33k to a 47k and the 470 pf cap to a 390 pf. Leave the values alone if you like the high mids. In fact you could drop the 33k slope resistor to 22k for even more punch. Try that with the 390 pf cap. I have used that mod before and it sounded quite good.
BRUCE is ABSOLUTELY CORRECT about not bypassing the cathode of V2(12AX7) at pin 8 if your amp uses a cathode follower circuit. The amp I worked on did not use a cathode follower, but that does not mean that yours doesn't! I followed the cicuit board traces on the one I worked on because like you, I didn't have the schematic. You can follow the traces to pin 8 of V2 and see if the tone stack is connected to pin 8. That will give you a clue as to whether your amp uses a cathode follower. You can also check for high D.C. voltage on pin 8 with a DVM to ascertain if it uses a cathode follower. Also a high value resistor, normally 100k will attach to pin 8 if it is a cathode follower.
If it is not a cathode follower you can bypass it with any value from .47mfd 25v electrolytic to 25mfd as a rule of thumb. You can use larger value caps, however 25 mfd or less seems to provide the best bass rolloff. Any value lower than .47 mfd will provide no appreciable difference in sound.
Hey Bruce, Thanks for your input. As usual you always keep us on our toes!
Jukka, Let us know what you decide,
|9/28/1999 2:19 AM|
I seem to have the same VC circuit Jukka is working with here. Mine is the 3112. I have traced the circuit and V3B is, in fact, a cathode follower.
If anyone would like a copy of my traced schematic, just e-mail me your address and I'll get a copy out to you.
|9/28/1999 5:39 AM|
We may be onto something here. The head I worked on had only three preamp tubes. The first tube was for the clean channel and the second was for the drive channel and then of course the third was the phase inverter. If V3B is a cathode follower in your amp then your amp and Jukkas are definately different than the head I worked on. Yours must have at least four pre amp tubes. I wouldn't mind a copy of the schematic for yours as there are a ton of Crate Vintage Club amps around here.
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