Tube amp guru, help me

[losthighway]

Hey gang,

A year or two ago I became the proud owner of a Dr. Z Jetta. While only an entry level Tone Dentist amp, I was thrilled to mark my entry into middle age with an overpriced boutique amplifier.

That thing rules! Yielded some of my favorite recorded guitar tones. Started making weird noises, then ringing, then I did some tapping and realized the power tubes (with a mere 8-10 hours use one them) were already toast. On a new amp!

The thing is cathode biased, so I figured, maybe it was a bad pair but I could just plug and play. I decided to try Sovteks this time instead (7591s aren't the easiest to find). Popped those in and hallelujah, sounds great. Then maybe 5-6 hours use the power amp tubes are microphonic again.

I can take this to a tech for sure, but I'm trying to get some perspective. What do you do with a cathode biased amp if it's destroying power tubes. You can't bias it. Seems lame to have to service a new amp with so little use on it, but c'est la vie.

[A.David.MacKinnon]

I had a similar problem with a client's zvex amp a few years ago. It was.pretty much new and the tubes were rattling and pinging like crazy. It was unusable for our session.
I'm no help beyond suggesting you find a good tech in your area. A good tech and tube amps go hand in hand anyway.

[losthighway]

I had a similar problem with a client's zvex amp a few years ago. It was.pretty much new and the tubes were rattling and pinging like crazy. It was unusable for our session.
I'm no help beyond suggesting you find a good tech in your area. A good tech and tube amps go hand in hand anyway.

Apparently they have a reputation for running hot and eating up power tubes, but I think this goes beyond that. It's entirely possible with all the talk of horrible quality control that I managed to grab lemon tubes twice in a row, but I'll feel like a fool if I stick a third pair in there for the same result.

[Scodiddly]

Obviously you could try to measure temperature on the outside of the tubes and compare that to a known reliable amp. A good amp tech should be able to measure the actual current through the tube to see if it's more than the tube is rated for - there's actually a little socket dongle that goes between tube and socket so you can measure for proper biasing. Another at-home test might be to look at the tubes in a dark room, to see if the plates are heating up or there's more of that blue glow than usual.

[floid]

7591's are a bit oddball, and the current versions i know of aren't much more than an approximation of the original spec. I'd assume a modern production amp would accommodate this, but, for instance, my Ampeg vastly prefers the EH over JJ variety, and it still required some circuit tweaks due to modern wall outlet voltages. One of the major weak spots of these tubes, in my amp, is the screen voltage.

On another note, cathode biased amps can be biased hotter or cooler via the cathode resistor. It's an iterative process, as the circuit does tend to rebalance/adjust itself, but for instance a 220 ohm kathode vs 470 ohm will be markedly hotter.

[roscoenyc]

Have you contacted repairs@drzamps.com ?

They are pretty responsive.

[Scodiddly]

7591's are a bit oddball, and the current versions i know of aren't much more than an approximation of the original spec. I'd assume a modern production amp would accommodate this, but, for instance, my Ampeg vastly prefers the EH over JJ variety, and it still required some circuit tweaks due to modern wall outlet voltages. One of the major weak spots of these tubes, in my amp, is the screen voltage..

Oh, 7591's? Those were unobtainable for a couple decades until somebody started making them again in Russian or China. I'd hope that a modern boutique amp would be based on the modern version instead of the original specs.

[Nick Sevilla]

Have you contacted repairs@drzamps.com ?

They are pretty responsive.

I second this. Get in touch with the amp maker.

This behavior is NOT normal.

[The Scum]

They apparently have a 3 year warranty, if purchased from an authorized dealer....they also void the warranty if anyone else has been inside it.

If you're within the 3 year window, that's the place to start.

[FNM]

I've learned a few times over the years that 3 is the magic number for tries to really verify whether or not you are just unlucky with tubes. If you are gonna have to ship the amp back anyways or whatnot, it might be worth the gamble.

another quick note; I've been fooled with microphonics before. I doubt this is the case, but make double sure. Are all 4 tubes bad or one from each set? If it were me, and I had two good mismatched tubes I'd try 'em out.

What's your wall voltage?

[losthighway]

What's your wall voltage?

120, I suppose. Horribly naive question but ........ why do you ask? (cause wall voltage affects performance and tube life?)

[FNM]

What's your wall voltage?

120, I suppose. Horribly naive question but ........ why do you ask? (cause wall voltage affects performance and tube life?)

Yes, higher wall voltage raises all the voltages in the amp. So if your screens are right at their limit (440v for JJ's for example) at around 120V, 125V would push them to like 460v. Not an issue in most amps, but in the right amp can definitely be an issue. I might be making this up but I think some transformers can still be wound for 115 too (not sure who Z uses, but I doubt this is the case). I know this because I used to live in a house with 127V.

[markjazzbassist]

most modern production tubes are garbage. especially the stuff EHX pumps out. They own a ton of the brands, i never buy their stuff, EHX, Sovtek, Tung-Sol, Genalex, Mullard, and Svetlana. AVOID THEM LIKE THE PLAGUE.

If you must have new production, go JJ/Tesla, i've found they have way better quality control. But i prefer to go Vintage NOS on ebay. They truly did make them better back then (because they were used in many applications, unlike today). There were a ton of different brands back then as well and anything american should be solid, or if you go euro try RFT. you don't need to get telefunkens, those will be pricey. I've done this for all my tube amps and preamps and they last a long long time and i've never had issues with them.

when i toured the new production ones would go microphonic fast due to wear in the trailer and moving in/out of venues. i bought vintage NOS ones and toured for years with them. They would then wear like normal, 2 years of heavy daily use and they'd need to be replaced.

[roscoenyc]

Have you contacted repairs@drzamps.com ?

They are pretty responsive.

I second this. Get in touch with the amp maker.

This behavior is NOT normal.

This is a new amp and that's not normal behavior.
The Jetta was only introduced in October of 2019.

Z's repair address is responsive.

[Recycled_Brains]

It's interesting, I've used EHX almost exclusively since Groove Tubes got sold to Fender, and they've always been reliable in my amps and studio gear. I've had the same quad in my Hamptone preamp since 2006-2008 (somewhere in there) and only this past month have they gone to shit (microphonic). Same with all my amps. Usually get at least 5 years of solid performance in a cranked Marshall JMP and Traynor YBA-1.

I do have a few nice NOS tubes. Telefunken 12AX7's, GE 5-star 12AY7s. In the context of studio gear I notice a bit of difference, but I seriously doubt I would notice in the context of a guitar amp. Not the way I do play anyhow. For me the risk-to reward-ratio, coupled with the crazy prices makes it too much of a gamble to justify the expense.

most modern production tubes are garbage. especially the stuff EHX pumps out. They own a ton of the brands, i never buy their stuff, EHX, Sovtek, Tung-Sol, Genalex, Mullard, and Svetlana. AVOID THEM LIKE THE PLAGUE.

If you must have new production, go JJ/Tesla, i've found they have way better quality control. But i prefer to go Vintage NOS on ebay. They truly did make them better back then (because they were used in many applications, unlike today). There were a ton of different brands back then as well and anything american should be solid, or if you go euro try RFT. you don't need to get telefunkens, those will be pricey. I've done this for all my tube amps and preamps and they last a long long time and i've never had issues with them.

when i toured the new production ones would go microphonic fast due to wear in the trailer and moving in/out of venues. i bought vintage NOS ones and toured for years with them. They would then wear like normal, 2 years of heavy daily use and they'd need to be replaced.