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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: EH 7591's posted by Jon Agner on July 27, 2002 at 02:18:22:
Jon,I bought my sets of EH 7591A from a seller which burned them in, before matching. I've heard that they do vary from tube to tube, quite a bit.
Also, I put them into a guitar amp, which bias them in Class "A" mode with a cathode-bound resistor.
They sounded cold with a bit of a scooped-out midrange with the stock OEM bias resistors (150 ohms, 5 watts) with a 25mfd/25 volt bypass cap. I dropped the resistance to 130 ohms, 5 watts with a 25 mfd/50 volt Sprague Atom bypass cap ---which raised the idle. This seemed to warm up the tone and give the amp more midrange tone---though not the warmth of the NOS 7591A.
You might want to try increasing the idle bias (especially, if you have FIXED-BIAS with an adjustable potentiometer).
Eventually, I went to different coupling caps in the phase inverter stage---and slightly lower voicing (to let more bass through), in the PI stage. This seemed to help the EH 7591A low to midrange response.
One thing I did notice---that blue glow. It's prominant, in my amp. And the tubes are so close to each other, due to the oversized envelope---that I got some strange pulsating EMF coupling between the two tubes. The blue glow would pulse out-of-phase between the two tubes and a very slight beating tone could be heard from the speakers---with no input and at high volume levels on the volume control.
This pulsing diminished with the higher idle point.
Anyhow, I voiced my amp like some 5881 amps which had original TungSol tubes. Then, new Sovtek were added. The tonal difference was a little like the change from the Westinghouse 7591A to the EH 7591A. The warm midrange tone of the TS 5881 is scooped out with the Sovtek 5881/6L6WGB.
To get the Bluesy tone in older Fender Bassman (and what is a Bassman which can't ooze Fender Blues-erosity?) some redialing of bias and revoicing of gain and PI stage helped a heap.
BTW---if you are judicious with your change outs in the voicing of the coupling caps (I went from 0.1 mfd/600 volt OEM to 0.22 mfd/630 volt Jensens in the PI stage) and 1% Riken resistors around the PI, plus the most symmetric Raytheon 6SN7WGT as the PI tube. Then, when I replaced the EH 7591A with NOS Westinghouse tubes---the amp still sounded great!
So, it seems that the EH7591 needs slightly higher biasing, in some amps. As symmetric PI section, which you can manage (including 1% resistors & best PI tube). And Jensen oil-filled coupling caps. You get close to the Westinghouse tone. But, still maintain the ability to go back to the Westies, without surgery.
Good luck, tell me how your amp turns out with some of these mods.
BTW---I've come across a few NOS SYlvania/Ampeg labeled & Westinghouse 7591A---so, I'm primarily using these tubes and keeping the EH in reserve. It's hard to beat the NOS tone, when it comes to this tube in my two Ampeg amps.
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Follow Ups
- Re: EH 7591's - steven oda 10:22:53 07/27/02 (2)
- Re: EH 7591's - Jim McShane 11:21:55 07/27/02 (1)
- pulsing "Purple Haze" - steven oda 14:39:20 07/27/02 (0)
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