Amp Techs - What to reasonably expect?

[Wasnex]

FYI, Somewhere between 40% and 60% of the tube amps I have bought had bad tubes or other problems. My advice when buying a tube amp. As a minimum, budget for a set of new output tubes and a bench fee. For an SVT this is about $450 - $500.

FYI, Perfectly matched tubes are not generally required since the SVT has separate bias controls for the push and pull side of the output section. The bias is adjusted to balance out the variances between the push and pull side, so hum cancels in the output transformer.

Every vintage-style SVT I have ever played through had audible hum...but AFAIK it usually originates in the preamp. My 69 hums more than my 87, but if I short the input of the 69's power amp, it's pretty much dead silent.

I believe the hum is originates in the active mid control. Set the mid rotary to noon and the hum is at it's minimum...turn the control up or down and the hum increases. My 87 has less hum than other vintage SVTs I have played through, but adjusting the mid control still has an audible impact.

 

[Grinderman]

FYI, Somewhere between 40% and 60% of the tube amps I have bought had bad tubes or other problems. My advice when buying a tube amp. As a minimum, budget for a set of new output tubes and a bench fee. For an SVT this is about $450 - $500.

FYI, Perfectly matched tubes are not generally required since the SVT has separate bias controls for the push and pull side of the output section. The bias is adjusted to balance out the variances between the push and pull side, so hum cancels in the output transformer.

Every vintage-style SVT I have ever played through had audible hum...but AFAIK it usually originates in the preamp. My 69 hums more than my 87, but if I short the input of the 69's power amp, it's pretty much dead silent.

I believe the hum is originates in the active mid control. Set the mid rotary to noon and the hum is at it's minimum...turn the control up or down and the hum increases. My 87 has less hum than other vintage SVTs I have played through, but adjusting the mid control still has an audible impact.

That's how it's looking with this amp regarding $500 for output tubes and bench fee.

The tech prefaced the news by explaining the balanced push-pull aspect and hum cancelation and that the tubes don't need to be perfectly matched. However, according to him, the Sovtek tubes that came with the amp are mismatched to the extent that the push and pull sides aren't canceling hum--low-frequency hum--as well as he would expect from a precisely matched sextet. The amp will work but not optimally--concerning how silent the amp could be.

I hope I'll have a chance to see or hear how much hum the amp is producing with the Sovtek tubes firsthand. If the Svetlana output tubes don't prove to make the difference, not mitigating what he believes to be the source of the problem, then I'd anticipate that's a preamp issue.



My '89 SVT-II (with good GT-branded GE 6550A output tubes!) is quite silent other than the fan and that's not even bad. However, as the low, mid, and high controls are turned up beyond noon they all will produce increasingly noticeable hum. The mid-frequency control does not affect the level of hum only the frequency of the mid-control hum. All of that I consider to be normal or to be expected.

One exception: Some days or times the high control hum is noticeably worse; it's noticeably present with the mid control set around noon. I have no well-founded idea why or what the variable involved with that.

I may take that one for a decadal checkup next as I do not know whether it's been inspected or serviced at any point within the last 20 years. I only know it was serviced in August 2003.

 

[Grinderman]

This could become a broad discussion but to start with, I'm wondering what others think is a reasonable range of time in which to hear back from an amp tech after dropping off an amp.

Being mindful that things happen, including holidays, and people can be very busy.

Conclusion

To sum this up now that I have the amp back and running it took about three weeks, the bill for their work was quite reasonable and I'm happy.

Reflecting on it, I'm glad I checked in with him at week two. I also realize some of these independent amp techs these days are getting up there in age, and their preferred means or device of communicating might be primarily by text message before voice phone calls and email not at all, in the case of this tech. They may operate in a very informal way as far as the business side of things. Be prepared for what we might assume or expect in a general sense to not be how they go about doing things. Not having dealt with the guy before that made me a bit nervous. In the end, it worked out fine and we had a good chat about a variety of things, including how he got into working on electronic gear for music.

If I go to him again to have an amp serviced--most likely I would--I'd know what to expect, how to handle his way of doing things, and I'd be comfortable with that; no worries to me.

I'm on the fence about naming the tech here, but if anyone might be purposefully curious send me a PM and I tell you.

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It turns out that taking this amp to have the output tubes put back in was the way to go because one of them in particular--of the original sextet of Sovteks the amp came with--was drawing significantly more current than the next highest one. To have the push and pull sides balance out the tubes had to be selectively organized or laid out.

After I went over there to see about it we agreed to keep using the Sovtek tubes for the time being and keep my fresh Svetlanas in reserve. Whatever additional low-frequency hum there is seems minuscule to me and completely drowned out by the cooling fan anyway.

 

[Wasnex]

I believe the hum is originates in the active mid control.

Was poking around in the SVT II service manual:

I suspect the same cause in my 69.

 

[agedhorse]

Was poking around in the SVT II service manual:

View attachment 5329757
I suspect the same cause in my 69.
View attachment 5329758

Yeah, that's not going to be fixed by the internet miracle solution of a "re-cap"

 

[Grinderman]

Was poking around in the SVT II service manual:

View attachment 5329757
I suspect the same cause in my 69.
View attachment 5329758

"some of the quieter units will not be improved by this change. It may be worthwhile to only change inductors on the noisier units."

I must be lucky on that one. I can't imagine any SVT being quieter than my '89 SVT-II. Having that side-by-side with my '78 Magnavox quickly and boldly put that into perspective.

 

[Grinderman]

Yeah, that's not going to be fixed by the internet miracle solution of a "re-cap"

So far, I've never had or needed a single cap replaced in an amp under my ownership, just a cracked cold solder once.