Princeton Reverb with 7591's?

[imaradiostar]

I've built an amp or two with EH 7591's and I think they sound great.

Have any of you out there tried to build a Princeton Reverb circuit with 7591's? I thought the lesser input voltage and bias requirements of the 7591's might make for an interesting twist on the Princeton circuit.

Jamie

[loudthud]

My only experience with 7591s was in a Heathkit stereo amp that was pretty close to the circuit in the back of an RCA tube manual with a 7199 as a driver tube. I was using NOS tubes. Power output was in the low 20s. Most people I talk to don't think the EH tubes are very close to the vintage examples. Some say the EH tubes are just re-pinned Russian tubes and they have lower gain than real 7591s. Do you intend to run a fairly high B+ like 7591s are capable of or just sticking with a more or less stock PR voltages?

Say a little more about the amps that you like the EH 7591s in. Are they something that would work just as well with 6L6s?

[imaradiostar]

I've used the EH 7591's in home-made guitar amps with between 400 and 500 volts on the plates. I've used 6L6's in the same amps and not been as pleased with the tone. My main gig amp is cathode biased with 425 volts on the plates and it sounds pretty awesome. If I recall correctly the grids are around 20 volts, much less than what was required to bias 6L6's. They require less signal and are easily driven by a Vox style LTPI with a large tail resistor and 100k plate resistors.

For the record, I believe you! I don't imagine either the EH or JJ sounds like the real deal form the good old days. I don't think that prevents them from being a good choice for guitar usage as they both have higher transconductance than a 6L6 and seem to require less drive to produce full output. I think they sound a little sweeter than a 6L6 too- not as harsh in the treble/upper midrange.

I was envisioning a Princeton circuit using an oversized Deluxe reverb or vibroverb PT with a solid state rectifier so it would have plate voltage in the range of 475 volts used with a 6k6 output transformer. I'd probably mod the PI to fixed bias and increase its supply voltage.

I have a Weber 6a14HP on my bench being built- I might have to jumper in some 7591's and an 8 ohm load (to mismatch primary to 8k) just to see how it sounds.

Jamie

[jrfrond]

The E-H tube is a decent sub for the original 7591A, but that's about it. It does NOT sound like the original.

THE most magical amp I've ever had in the shop, a Premier Twin 8, had the original 7591A installed in it. This amp had all the mojo one could ever want, that is until I subbed in a new E-H tube. It lost the creamy overdrive tone and smoothness completely. Found the same issue with other 7591-based amps. Hey, it is what it is! We ALL know that there are irreplaceable NOS tube designs, but if you can't afford the real McCoy.....

[imaradiostar]

Now you guys are just ruining my Christmas!!

I'm sad to hear the new ones aren't as good but I still like the way they sound in my amp. Like so many things I guess the answer would be to build it and see how it sounds.

I really wish they could get the new production 7591 correct. I think they were such a neat tube- high gain and power but different from an EL34.

jamie

[dchang0]

Noticed the same thing with a NOS GE 6BM8 tube versus the EH 6BM8. The GE is cleaner, tighter, and has a flatter response. The EH was acceptable but noisy and microphonic. I returned it, but considering that EH is the only maker of new 6BM8s that I know of, I'll definitely be using them in the future, saving the $40 vintage and $20 NOS tubes for recording sessions.