Supro 1622RT TremoVerb Hum Problem

[Daniel S]

I really don't want to have to use Electro Harmonix again.

Is this a good deal?

https://www.ebay.com/itm/Pair-of-RCA-69 ... :rk:8:pf:0

The hat/top on that one looks crooked. Is that ok?

[pompeiisneaks]

The crooked top is typical in glass, they have the glass glowing hot when the twist them to separate from the main glass tube.

As for the quality of the tubes in general RCA are normally rock solid, I don't know tons about NOS tubes, but i hear ebay can be hit and miss with some sellers selling 'picks' as NOS.

YMMV with this seller. The tubes, if they are as represented, will probably be amazing.

~Phil

[xtian]

Great. Then, as Phil said,

That leaves failing or mismatched power tubes, filter caps

Try a new, matched pair of power tubes to troubleshoot first issue.

Clip in a new 47uF/500v filter cap at your reservoir to troubleshoot second issue.

[martin manning]

The crooked top is typical in glass, they have the glass glowing hot when the twist them to separate from the main glass tube.

It’s the getter flash that is crooked. Might be cosmetic, or indicative of some misalignment. EH tubes are generally pretty good. The ones coming out may just be worn out, and possibly abused.

[Tony Bones]

I really don't want to have to use Electro Harmonix again.

Is this a good deal?

https://www.ebay.com/itm/Pair-of-RCA-69 ... :rk:8:pf:0

The hat/top on that one looks crooked. Is that ok?

I think that a pair of 6EM5 or 6CM6 should be a drop-in replacement. The only difference is that the 6973 has the screen grid coming out to both pins 2 and 8 whereas the 6EM5 and 6CM6 bring it out only to pin 2. I'd leave pin 8 unconnected with those; sometimes they connect unused pins to support rods. I've used and like the 6EM5. Not in a Supro, but in other applications, and they can be had NOS for dirt cheap.

[drew]

The crooked top is typical in glass, they have the glass glowing hot when the twist them to separate from the main glass tube.

It’s the getter flash that is crooked. Might be cosmetic, or indicative of some misalignment. EH tubes are generally pretty good. The ones coming out may just be worn out, and possibly abused.

And/or the amp seller could have raided the original tubes and subbed these in from his junk pile.

[Daniel S]

Thanks for the replies everyone. I think the safe route is to order a new matched pair of the EHX 6973's just to confirm the problem is solved. I can always buy NOS later. I'll probably order a matched quad of 12AX7's and a new 12AT7 to round it out. One of the tubes that came out of the back - where you can't see without pulling the chassis - was a no name clear weird looking tube that only had "12AX7B China" stamped on it. The insides looked like a totally different construction than the EHX's.

Also, the EHX 12AX7EH's had a variety of numbers (dates?) on them:
07 10
09 10
07 09
14 06

The 6973's were both marked: 15 01

Anyways, I'll order the tubes and hopefully get them sometime next week. Thanks again for all the input.

Edit: Are there any reputable tube sellers associated with Amp Garage? Somebody who's known to the community?

[xtian]

There is no point buying matched-triode or matched sets of 12a?7 tubes for guitar amps. Matched POWER tube, yes.

I get tubes from tubesandmore.com, usually JJ which have served me reliably.

[martin manning]

Matched triodes can be a good thing for a PI tube.

[pompeiisneaks]

Matched triodes can be a good thing for a PI tube.

I thought this was only if you have the ability to balance the PI with a PI Pot in the first place no? Most amps don't have perfectly balanced halves of the PI section anyway, differnces in anode and cathode resistances etc.

I.e. a well balanced PI tube is only good if you can also balance the halves of the actual tube resistors etc. ( a la dumble tweedle dee or ods)

or am I nuts?

~Phil

[martin manning]

The reason for the miss-matched plate loads on a LTP is to balance the outputs. That works as long as the triodes are matched ;^)

[pompeiisneaks]

The reason for the miss-matched plate loads on a LTP is to balance the outputs. That works as long as the triodes are matched ;^)

You mean like the typical marshall 80k vs 100k mismatch? (as well as the dumble 110 vs 120k plus pot), that's the point I was trying to make I think, on other amps the typical PI has two 100k instead so there's an expected mismatch.

right? does the supro PI have the anode resistor mismatch for the PI as well?

~Phil

[martin manning]

Miss-matched loads on an LTP inverter is far more common than not since Fender did that in the Tweed days, and Marshall copied it, of course. The need for miss-matched loads depends upon the tail resistance which, if high enough, gives acceptable balance with equal loads. But, again, that assumes identical triodes. A lot of Supro amps have paraphase inverters, and it's more than likely they were also designed assuming identical triodes in the PI tube.

[pompeiisneaks]

Miss-matched loads on an LTP inverter is far more common than not since Fender did that in the Tweed days, and Marshall copied it, of course. The need for miss-matched loads depends upon the tail resistance which, if high enough, gives acceptable balance with equal loads. But, again, that assumes identical triodes. A lot of Supro amps have paraphase inverters, and it's more than likely they were also designed assuming identical triodes in the PI tube.

very cool, also, paraphase would be very prone to needing balance I'd guess then right?

Thanks for the edumacation

~Phil

[Tony Bones]

It's academic, but I'll argue that with an infinite tail resistance (i.e. a current source) the balance in a LTP depends on the plate loads and not the tubes themselves. The tubes could be mismatched, but if the plate resistors are equal then the balance will be there. Admittedly, the tail resistance in a typical Fender or Marshall LTP is not infinite, but it's higher than might be thought at first because the unusual NFB arrangement bootstraps the tail to some degree. I'm not sure how to even go about calculating the degree of bootstrapping, but I understand the concept to be correct. In any case, the effective tail resistance is very large compared to the resistance looking up into either cathode in the LTP. With absolutely no measured data (why have I never measured this?) I'm willing to bet lunch money that the 82k/100k plate resistors do more to hurt than help the balance in Fender/Marshall PIs.

Of course none of this has anything to do with the hum that the OP is experiencing. That might be due to mismatched output tubes.