Twin Reverb PT Surface Temperature

[93DMTX]

Hello,

I've been rebuilding a 1970 SF Twin. Replaced all the electrolytics, converted hum adj to bias adj, replaced power amp grid and screen resistors, new baffle board, replaced ripped Oxfords with a pair of used swamp things, new quad of 6L6s, reglued and pinned cabinet. New tolex and grill cloth are coming. The thing is sounding really good...albeit too loud for me. Virtually no odd noises after the screen and grid resistors.

At some point in its life the PT was replaced by what appears to be a '65 TRRI xfmr. It seems to work fine though since it does not have a CT for the high voltage, the rectifier has been converted to a full bridge. It is giving me a plate voltage of ~450 with a quad installed. Kinda high but OK I guess. Got it biased kinda cold though not nearly as cold as it was with the hum adj.

Anyway, the thing I've noticed is the PT gets almost too hot to touch after 30 minutes or so. I've never had an older twin like this so I don't have a reference point but it's way hotter than my other amps (Shiva, DSL100, Rectoverb).

I'll have an infrared thermometer tommorow to take some readings.

Just wondering if anyone had thought on how hot a twins PT should get. BTW, it heats up in standby so I'm assuming it's primarily the heater current. Seems like it should be ok though since the reissue has the same tube compliment.

Thanks,
Doug

[Tom Phillips]

My experience has been that the classic Twin Reverb PT will get too hot to comfortably handle after the amp has been on for a while. Typically the core and end bell will measure about 145° F when the ambient temp is around 70° F.

I think that it usually takes longer than 30 minute to reach a stable temp at idle but 30 min could be normal if the amp is being played loud. In that case it would probably get a little hotter than 145° F. In addition to IR and core loss heating the PT is really close to two of the power tubes so the tubes' physical presence will add heat to the PT.

You can use your IR thermometer to determine if your temps are close to the above. You can also pull all the tubes and the pilot light a measure the PT temp rise under no load conditions. The no load temp rise for that TP is typically about 20° F
above ambient. If the PT gets really hot under no load conditions it indicates an internal short.

Let us know what you find.

Regards,
Tom

[MWJB]

93DMTX - How much idle current are the power tubes drawing? How many ma's is "kinda cold"? This may not be an issue (in light of Tom's detailed response) but plate current is what causes the most heat in an amp. Many parts typically run hotter than is comfortable for a human hand (~65C-70C?).

450vdc on the plates is only 10vdc higher than schem (440vdc at 120VAC line, -50vdc bias).

[93DMTX]

Tom,

Thanks for the info. I'll get some temperature reading this evening and post them back.


MWJB,

I guess I'm not really thinking its the plate current at this point as the heating occurs even in standby.

I'm biased at about 25 mA cathode current per tube. The tubes were idling at less than 15 mA before making the bias modification.

This an AA270 (non master). The schematic shows the plate voltage as 405VDC hence my comment. I do have usually about 122 VAC at the wall and I do not have access to a Variac.

Thanks,
Doug

[MWJB]

Hi Doug,

Twin voltages varied quite a bit over the years, anything from a shade over 400vdc to 525vdc might be "normal", depending on year/wall AC/bias conditions.

Indeed, 25mA per tube is "kinda cold" ;-) Don't mind me, I'm just a pedant and always like to have a number, rather than a judgement ;-)

Pull out any unused preamp tubes, see if the reduction in heater current draw has a tangible effect? How are your heater voltages? It does seem strange for heaters alone to cause high PT temp, but it's worth pointing out I have no first hand experience with guaging the RI PT (only original & Hyboer) in this respect.

[bob p]

^ I have a Super Twin Reverb that idles with a B+ of 530 VDC on a modern 125 VAC input voltage when biased at the recommended -60VDC. I haven't measured the iron's temperature, but it's definitely warm enough that you don't want to put your hand on it ... and that's at idle, not under load. I'm thinking that this amp is eventually going to get a fan.

[Tom Phillips]

…I'm biased at about 25 mA cathode current per tube.

So, at your 450V plate voltage, that is 11.25 W dissipation. (a little less if you want to correct for the screen current contribution)

Heresy Alert
I think that 25 mA @ 450V sounds good. It not only sounds good to me, it looks good on the scope AND customers often pick that setting when I tweak the bias point during sound tests and give them several settings to choose from.

Obviously I don’t buy into the 70% rule which came from ...? Actually, I don’t know where that came from but it sure is bantered around on the internet and in “guru” books a lot.
YMMV
Cheers,
Tom

[93DMTX]

Ok, here is the first pass...

Tubes pulled (pilot installed - I forgot it )

The amp had been off for approx 20 hrs.

Initial - Core: 79F / Top Bell: 79F
+15 Min - Core: 90F / Top Bell: 89F
+30 Min - Core: 100F / Top Bell: 99F
+45 Min - Core: 112F / Top Bell 105F
+60 Min - Core: 113F / Top Bell 105F

Heater no tubes 6.96VAC
Heater with all Pre's (6) installed 6.80VAC
Heater with all tubes 6.53 VAC

================================

All the tubes are back in, unit in standby. I'm just going to find where it stabilizes this time.


At this point I'll say that my fingers are not well calibrated. I'd have guessed 113F was at least 150F.

=================================

Tom, I think at 25mA this thing sounds just like it should. Personally I think the Swamp Thang's are a good fit for this amp too.

I'll try to attach a pic of what it looked like with the Oxfords

[Tom Phillips]

All could be normal but my gut feeling is that the PT is heating up faster than average under no load and I don't think that the pilot light would be cause that.

Other tests you could do.
1) Measure the magnetization current. I.e. that current the primary draws at 120V with no load on any of the secondaries. I'd expect it to be ~ 250mA and stay near that value as time passes.
2) Make sure there is really no current flowing in any of the secondaries. Does it reduce the primary current if you lift the red HV secondaries?

Looks like that amp has had a rough life.

[93DMTX]

So after 30 min with all tubes / amp in standby seemed stable at ~135F.

Then going full on, but no signal after about 30 min more stable at ~150F.

BTW, all temperature measurements are on the side opposite the tubes.

May try making the primary current measurements tomorrow.

Thanks again for the suggestions, you guy's sharing your knowledge and exerience is so very helpful.

Looks like that amp has had a rough life.

Yep. But it was basically a gift and now I'm thinking of it as a rescue.

[drewl]

The power trans in my '67 Pro Reverb also gets very hot FWIW.
I installed a Weber solid state rectifier plug in to have it cooler for road use and can pop in the rectifier tube if desired.

[Jazz P Bass]

With 4 6L6 tubes, biased at a reasonable idle current, the Twin is probably drawing 100-120 watts from the mains.
That heat has to go somewhere.(ie: power transformer)
Just a thought.

[Tom Phillips]

Doug,

I made some measurements on a Fender P/N 022756 PT last night. This is the transformer that Fender used in the Silverface Twin Reverb & Showman. It was a bare transformer sitting in the open on the bench. Primary voltage was adjusted to 120V and the measured magnetization current was 270mA with all secondary windings open circuit. I let it run over night and with an ambient shop temperature of 60.5°F the core temp stabilized at 89.5°F. This 29°F temperature rise is consistent with your measurement.

As others have confirmed these transformers run hot to the touch in the amp. All the temp readings you have posted seem to indicate that your transformer is operating normally. No red flag there for your rescue amp.

Cheers,
Tom

[93DMTX]

Tom and All,

Thanks again for taking the time to help with this. I believe that the PT is probably OK as well now.

For future reference this transformer is labeled as a P/N 037610.

Over the weekend I'll lift the secondaries and post the primary current in case anyone needs it for future troubleshooting.

=========================

On another note got my tailed fender logo in yesterday (off ebay) and the tolex and grill cloth should be in later this week.

I'll post pictures of the rescue back here once it's done.

Best Regards,
Doug

[Tom Phillips]

...On another note got my tailed fender logo in yesterday (off ebay) and the tolex and grill cloth should be in later this week...

Doug,
Some tips: Take note of where the original logo screw holes are positioned on the baffle board so you can mount yours in the proper position after the re-grill. Remember to add the surround spacer boards to your new baffle as well as the backing for the logo before you install the grill cloth.
Tom