 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
24.15.32.233
In Reply to: RE: Scott 340A adjust to run newer 7591 question posted by DannyR on April 03, 2012 at 12:35:22
You don't need to do anything. The 330K resistors are in parallel with the 270K going to the negative supply via the balance pot. In parallel the total resistance is under 200K.
I would recommend installing a 10 Ohm resistor in each power tube cathode (so you'll need 4 of them) so you can adjust the balance better. There is a procedure for the Scott, but I like being able to measure directly across the resistor and use Ohm's law to get my current. And working in the cathode circuit is safer too!
Follow Ups:
Jim,
I've seen reports that things can be VERY tight in Scott units and that the large bottle of the EH7591 may not fit. :> ( Using "firecracker" JJ 7591s is (IMO) out of the question. So, if things are really tight, switching sockets and installing EH 7868s seems to be best.
Eli D.
There's plenty of room for EH7591 in a 340A.....
"There's plenty of room for EH7591 in a 340A."
Outstanding!! :> D
Eli D.
Hi Jim, Okay great. So a 10ohm resistor between pin 5 and ground. Do you think that Electro Harmonix 7591's will hold up in the amp without any other reworking?
Thanks
They are VERY GOOD TUBES!! Don't get discouraged by the "if it isn't NOS it isn't any good" crowd. They are reliable, and they have excellent tone!
I saw another poster mention individual bias pots. If you are up to the task that's a great idea. 1/2 watt pots are fine. With the pots and the 10 Ohm cathode resistors you can adjust the bias far better than a stock 340A could be adjusted.
Have fun!
Thanks for help Jim. What wattage 10ohm resistors from pin 5 to ground would you use? 1/4watt? Thanks
No, not 1/4 watt! You see people say that all the time - because the STATIC (idle) dissipation is very low they'll say use 1/4 watters. But what about when music is being played - what happens during crescendos, etc.?
I use 1/2 watt minimum, for your amp I use 1 watt metal oxide film so they blow quickly in case of an overload but don't fail on loud music. You could probably get by with 1/2 watt in a 7591A cathode, but I prefer 1 watt.
For whatever stupid pennypinching reason, Scott put DC balance pots on the 340A but no bias pots so you can't easily adjust bias...which you should be able to do especially if you want to run precious vintage 7591's. I added them to mine....IIRC ~250K pots inserted between the bridge and the wipers of the DC balance pots. This allows the adjustment of bias to each output pair...which can be subsequently balanced with the DC balance pots.My adjustment routine would be to attach my DVM leads to the two cathodes of the 7591 pair (no signal; clip just before the 10 ohm 1/4w resistors)....and adjust the dc balance control for zero voltage to balance the pair. Then clip one of the leads to ground and adjust bias control to get voltage of ~300mv across the 10 ohm resistor which corresponds to ~30mA current through each tube in the pair. Repeat for the other side.
Edits: 04/03/12
Hi Dadbar, that looks great. Thanks for sharing the pictures and idea. What's value is the resistor that you have soldered across the new pots and will 1/2 pots do the job?
I don't recall the resistor value off hand....it was a little while ago when I did that. If memory serves me correctly I needed a 100K or 250k pot but only had 1M (1/2 watt linear) on hand so I used another shunting resistor to reduce the pot value a bit. If you already have a lower pot value you don't need the resistor.
Edits: 04/03/12
Great. Thanks again Dadbar for the help. I greatly appreciate it.
Post a Followup:
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
