More guitar amp questions

[dbishopbliss]

I have the schematic for the amp I want to build, but it does not have any voltages or currents listed.  Is there a way to reverse engineer the voltages from a schematic?

Here's the schematic.

[elcraigo]

Sure - just about anything can be reversed engineered with various degrees of pain.
Can be somewhat of a chore to do by hand. I personally would redraw the schematic electronically and create a spice model, and then run some simulations.
But, on the same website you linked, there is a version of the schematic with voltages.
It is Plexi6V6.gif  (as opposed to Plexi6V6c.gif doesn't have the c in the drawing) I did not look close, but the 'c' might be a revision, so there might be some differences between the two.

[dbishopbliss]

Doh!

I didn't bother looking at that schematic because MH said he updated errors on the original.  I imagine I will be in the ballpark using those values.

Question 2 has to do with my choice of output transformers.  According to the designer, MH, you can use the transformers intended for the popular "18 Watt" Marshalls which are based on EL84's.  The primary impedence for these transformers is 8K.  MH was using a 6K6 primary transformer.  In theory, the 8K will be more linear so it would have less distortion.  Since this is a Marshall-style guitar amp, I want the distortion.  Do you think there would be a significant difference in the sound using 8K vs 6K6? 

Also, I have noticed most output transformers out there are made using M6 laminations.  I have read that many of the old Plexi-style amps actually used M19 steel.  If I understand correctly, this is a lower grade steel and is not recommended for audio use because it will saturate more easily.  Again, I'm building a guitar amp and want the distortion... I would prefer the distortion at lower levels too.  I have found a 6K6/M19 transformer intended for Trainwreck-style amplfiers. Trainwrecks are known for their great tone, so I'm thinking this would be a good match.  Do you think using the 6K6/M19 transformers would make a significant difference from the 8K/M6?

[Doc B.]

There are lots of different kinds of distortion. I'm not so sure that the distortion from OT core saturation is necessarily a major ingredient in the sound you might be trying to get. If you want distortion at lower levels you usually need to have multiple stages so you can reduce the master output and overdrive the intermediate stages. Unfortunately in most amps this creates triode distortion rather than the usually more desirable pentode distortion you get by overdriving a pentode output stage. A better approach might be to use a less sensitive speaker and/or an L pad brake so you can overdrive the output stage.

A good book that deals with a lot of this stuff is Gerald Weber's All About Vacuum Tube Guitar Amplifiers.

[2wo]

You might try one of the small Hammond, what is it? 125 series? Lots of taps. might find what your looking for...John

http://angela.com/hammond125seseriessingleendedoutputtransformers.aspx