Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on January 23, 2017, 05:13:45 pm
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I'm not interested in a PPIMV, as i amd happy with the PRE PI MV in my 6V6 home brew. But trying a PPIMV in not only this but several other similar amps i have built i run into this same issue. the other amps were EL34 amp but they all have almost the same preamp and same circuit exactly from the last preamp stage thru the PI as a 2204 as i linked below. So last preamp stage>cathode follower>tonestack>MV>PI is pretty much exactly as in the schematic below except for the PSU rail, but i have tried many variations of that in this and the other EL34 amps i built with droppers from 15k to 4.7k and filters from 22uf to as much as 80uf, chokes sometimes, other times a resistor. And the issue is always present which makes me wonder what could cause it and how others don't have this issue.
So without further ado, the issue. The times i have tried PPIMV's i have tried crossline and the dual pot designs and the same issue happens every time. Turned up to even home or bedroom volumes the tone is horribly ratty and thin like it;s going thru the worlds worse sounding fuzz box. At first i thought too much signal but even with the gain very low it still happens. Maybe the preamps all have way too much gain, i dunno. But while i DO have a regular gain stage in place of the 2204's cold bias stage, it;s not crazy at all. And i have the first 2 stages bypassed with .68uf and 2.7k on V1A and 4.7k on v1B. I can't imagine with the gain on say noon with those specs the PI should be getting slammed. And i have even tried it leaving the PRE PI master in so i could turn that down. And with the PRE PI master why wouldn't i get that horrible sound when thats turned up with no second PPIMV? Any ideas? As i siid i want to keep using a PRE PI master, but this has me wondering.....if theres a design flaw it may be affecting the amps tone to some degree.
2204: http://schematicheaven.net/marshallamps/jcm800_lead_mstvol_50w_2204.pdf (http://schematicheaven.net/marshallamps/jcm800_lead_mstvol_50w_2204.pdf)
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I've only used/tried the crossline and don't care for it. I was talking with my Gigging Son and he picked up a siverface twin and said he liked the MV on it, that's probably your best bet, go to guitar amp store, play all the PPIMV amps they have, hunt down schematics for the one's you like and :dontknow:
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I'm not looking for a better MV. I like my pre PI master. What i'm trying to figure out is why it sounds horrible (unusable really) every time i have tried post PI masters. (of several types, not just crossline) because if theres a design issue causing it then it may be affecting the tone of the amp no matter what the master volume type.
Also, what kind of AC voltages am i looking for at the pre PI master's output? Maybe that will be telling. With gain around 1:00 and the master around 9:00 i can hit a chord and get around 20vac. Seems like a lot but i really don't understand.....if it IS a lot, then why don't I get that nasty tone with the pre PI master at 9:00 but if i install a second master post PI then it gets that nastiness no matter where it's set and even with the PRE PI master still at 9:00 or ANY setting for that matter?
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I have tried many and I have found usually you cannot dial down a PPIMV very much and with an amp with presence,not much at all or it gets dull. The LARMAR master that uses dual 250K pots and the 2m2 resistors to maintain bias usually will only take off the some of the overall output. Limiting this master to about 100K is about all I find to maintain a nice tone and keep the highs.
If I have an amp that I do not mind sag, a blues amp with cathode bias the light bulb string used in Samamps works great. For a Marshall or any amp with a faster attack the JCM800, 2204 preamp is the best IMO. It has a better sound overall.
From the Trainwreck pages, the dual 500k may be worth a try. I ended up using one of these on a 5f6a Bassman clone and it is the best I could find. I tried many