Tube Amp Archive

Here we are. This is a JCM800 preamp. We can say the father of all three stages plus CF preamps. http://www.tremolo.pl/Przedwzmacniacze/Marshal-2204prem.gif Thanks Roberto, I'll study that schematic and do some research. As far as my preamp goes, I think I've probably reached the limit of my abilit...

roberto I see that he,s probably struggling with understanding what a 3 gain stage with CF is, Im at work or I would put a schematic up for referance. Bill ryanpg I would not use a 12at7 to cut down the gain get a 5751 or 12ay7 .I dont like the sound of 12at7 when it distorts. Just my personal pref...

If you don't have the coupling trafo anymore, you can simply convert to a three stage plus cathode follower (split loaded to have a low amplitude signal for the rest of the preamp), and remove the two diodes between tubes and ICs. Remove also the grid stopper to stage II. So, remove D9 and D10 comp...

I've settled on putting a 0.001μF cap across R7 for now. It allows more highs through. With presence around 8 and tone around 3-4 it's fairly bright. I also put a 12AT7 in T1 just to mellow out the distortion. Things I've tried but reverted: - A 0.022μF cap parallel with that 1M resistor, ...

roberto wrote:Hi, I've your PM asking my help, but cannot read all that.
Try to resume what you need, or nobody will reply.

Hi roberto, My apologies. I'll try to be concise.

There's a real lack of high-end/treble, it's very muddy. I'm not sure where I'm loosing high-mids and treble.

To summarize my understanding of the transformer conversion: A 100k plate resistor has been added across the primary where the transformer was. C60 .0047μF 400v (which was R60) is now a coupling capacitor. It also acts as a filter (smaller value, less bass frequency). 1M attenuating resistor ad...

Aha! The mystery of R6 and R60 has been solved!

R6 - Hiding under the 0.001μF cap is a little 300k Ohms resistor!

R60 - what I thought was a .0047k resistor, is actually a capacitor! So R60 IS my coupling capacitor, and R6 IS a plate load resistor in parallel with a cap.

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Teemuk, thanks for the detailed replies and instruction! I appreciate you going through most of my questions and confirming/correcting each. Struggling with understandingf the most basic and elementary things cast a shadow of doubt to any information you provide and basically puts you into a positio...

It appears my preamp (as seen in the pictures) deviates from the schematic in many ways: - The coupling/output transformer after the tube stage has been replaced with a couple resistors. - R60 has been replaced with a capacitor - R61 is removed - R23 is removed - R24 has been changed from 1K to 220k...

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The photo below is not of my unit. This one appears to have the transformer still in place and no push-pull gain.

And finally the schematic for version 4 (the photos of mine are V3)

Continuing on with pics.

Hi I just got a Hafler T2 guitar preamp. I've taken some gut shots and attached a schematic.