Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on August 21, 2026, 06:08:00 pm

Title: Comparing 2 of the same amps but find differences
Post by: pbman1953 on August 21, 2026, 06:08:00 pm: I'm working on 2 of my Sunn 2000s amps. One amp has a solid state section inside the chassis and no tube rectifier sockets. The other does have sockets.

The one with the SS rectifier doesn't have the 220uf cap and resistor (1.5k) off pin 8 to ground. It has only a 1.5 k resistor. The other one does have the cap and resistor.

The schematic shows that the cap should be there

What could be the sonic difference?
Title: Re: Comparing 2 of the same amps but find differences
Post by: sluckey on August 21, 2026, 09:07:01 pm: Make it look like the schematic. You'll have a big bass boost and you'll have less filament hum.
Title: Re: Comparing 2 of the same amps but find differences
Post by: pbman1953 on August 22, 2026, 07:46:47 am: Quote from: sluckey on August 21, 2026, 09:07:01 pm
Make it look like the schematic. You'll have a big bass boost and you'll have less filament hum.

On it , thanks!

Another question. these 2 amps are very close to the schematic in most areas. . My first one, the one I set up to use the KT-120's , has a bit more low end than the second. Is that the difference between the older Tungsol(USA) 6550's and the newer JJ 6550's? Both sound good but the first one has more frequencies in the low end that second doesn't. Another difference is that the first one has increased storage capacitance ( 110uf) + 50uf) in the first 2 stages . The second one is close to stock values (40uf + 23uf)
Title: Re: Comparing 2 of the same amps but find differences
Post by: pdf64 on August 22, 2026, 08:28:06 am: Quote from: pbman1953 on August 22, 2026, 07:46:47 am
... Is that the difference between the older Tungsol(USA) 6550's and the newer JJ 6550's? ...
Seriously?
From what you write, there seems to be significant differences in the circuits of the 2 amps, at the input stages and in the HT supplies.
Swap over the valves in the same amp if you want to see what difference the valves make.
Title: Re: Comparing 2 of the same amps but find differences
Post by: sluckey on August 22, 2026, 08:32:40 am: The two circuits are different. You should expect the sound to be different. You're being too nitpicky.
Title: Re: Comparing 2 of the same amps but find differences
Post by: pbman1953 on August 22, 2026, 08:57:54 am: Oook, maybe I am.

Once last one. Due to age, do these diodes need replacement?

In the blocked direction, with a meter on the diode setting, they are all OL

Here are the readings in the on the output side-.573 , .566, ..573, ..578
Title: Re: Comparing 2 of the same amps but find differences
Post by: acheld on August 22, 2026, 09:48:11 am: Those just tested good, so no. I generally don't replace good diodes for age alone.
Title: Re: Comparing 2 of the same amps but find differences
Post by: pbman1953 on August 22, 2026, 09:49:05 am: Great, thanks!