Phase inverter variations in the 70s (Steve Grindrod)

[Roe]

Factory schematics from the late 70s shows that Marshall/Park tweaked the phase inverter to improve balance under overdrive (so that the first power tubes won't redplate and the amp will ghost less). Instead of using two 1m resistors in the phase inverter Marshall/Park used 270k/1m, then 330k/1m and finally 390k/1m (with 100k @ 4ohms negative feedback).

See the following schematics:
- 4140 preamp/twin channel reverb amp
- 2150 preamp/master volume powercel combo preamp
- Park 1210 preamp
See Doyle History of Marshall, pp. 237-8 and 247. See also:
http://vintageamps.com/plexiboard/viewt ... 22#p780322

[Roe]

so people care about ghosting and redplating but not about possible solutions?

[Carbia]

Nice info!

[stratonenator]

Does this effect the overall gain of the amp or will it simply help with balance and ghosting?

[Roe]

Does this effect the overall gain of the amp or will it simply help with balance and ghosting?

The latter, although the former will be slightly affected if you use a PI input cap that is 2200pf or smaller

[bill bokey]

Interesting, I never noticed that !
Thanks for the info, Roe

[Roe]

The best option seems to be balancing the plate resistors on v3, preferably with a balancing pot like dumble used in conjunction with two resistors (one at each leg). This will really help with balancing the driver, avoiding redplating on one side and minimizing ghosting

[Roe]

Really, the best option is do like Traynor and Dumble did in order to balance the phase inverter and to get max power. Traynor used a 1meg balancing trim pot instead of the two 1meg resistors, sometimes with a small resistor at one leg. Duble used a trim pot between the two plate resistors

[stratonenator]

Roe, I can't seem to find a schematic of what you're referring to. Do you have either the traynor layout or the dumble one?