Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on February 23, 2024, 09:51:48 am
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A friend has asked me to refurb his 1939 Recording King amp, that was partially refurbed a few years ago. All the caps and about 1/2 of the resistors were replaced. It needs 3 new pots, and all of the remaining caps are about 30% out of spec, so I am going to replace them too. This is around a 17 watt amp with dual 6L6's. It's totally point to point wired.
I have a question about the best method to use in replacing the resistors. Some of them are pretty accessible and not connected at a joint with other components being wired in, like at a lug on a tube socket. But there are others that are soldered together in a "bundle" of several components soldered together.
Is it better for some, or maybe all, of these replacement resistors to cut out the old resistor and use a J-hook to wire in and solder the new resistor leaving the original lead intact, or is it better to remove the old, original solder joint and resistors, and re-solder them all back together with a new solder joint?
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what's the end goal?
you can get an 'ol classic runnin and call it a survivor car, or you can do a frame off restoration.
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To clean up the sound.
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Hi-rez pics please.
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Here are a couple of photos.
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"Is it better to make a clean, workmanlike job of it, or just scab-on some new parts and call it good?"
Can you tell how I might feel about it? =)
Really, though. This thing is 85 years old, with a majority of parts failed, failing or already replaced. In many cases, the parts they used back then weren't nearly as stable as what we can get today.
I'd be considering a near shotgun-rebuild, saving only the best preserved and most stable parts.
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Here are a couple of photos.
Nope, they don't make 'em like that no more!
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Here are a couple of photos.
Are those the smallest pics you have? :icon_biggrin:
Really, you ain't gonna hurt the looks of that amp. Just do what is easiest and call it done. :thumbsup:
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Agreed, the appearance is not salvageable. I just figured if I was going to replace these components, I would at least want to give it a fighting chance to sound better with the new components. Maybe it will sound better anyway with new components, regardless of how they are installed.
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Maybe it will sound better anyway with new components, regardless of how they are installed.
The quality of workmanship shouldn't have much if any effect on the sonic qualities, so long as you make good solder joints.
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I would at least want to give it a fighting chance to sound better
do ONE!! socket, test
do 2nd socket, TEST
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the more you do at once the better chance Murphy comes along and ruins your day
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Good point Shooter.
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Hopefully you can find an old schematic... Normally those old ones will have diagrams with voltages at every pin, and that would be a place to start, assuming it's running safely atm.
Normally I've found it's beneficial, if there's one bad part in a mess of spaghetti, to replace everything in the mess of spaghetti because likely it's going to need to come out anyways to replace the offending part. On the other hand, if the parts read within spec on your meter, and they've been there for the last 90 years, they'll probably be ok. I have yet to find a bad potentiometer from that era, but maybe I'm lucky. I've found quite a few that needed several cleanings though.
Be wary of any original capacitors, as well as any of the large, low value resistors, as they are likely the first to shit the bed. Normally the only capacitors I'll trust in an old radio is ceramics and micas, in that order.
The hook up wires can often be the biggest problem in those old amps, the old rubber, plastic, fabric, whatever often gets so brittle that it just flakes off, and if you encounter that problem you'll be in for a fun and educational time.
What is it doing that it doesn't sound right?
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Yes, I do have a schematic. No voltages anywhere on it though. Its tone pot was making all kinds of scratchiness, hiss, hum and feedback. So I replaced that with a pot I happened to have, and that seems to have fixed that problem. But there’s still some low level hum. All 3 of the pots were way out of spec by at least 30%, and all of the caps and about 1/2 of the resistors have been replaced. So I’m going to replace the remaining pots and the remaining resistors, and hopefully that will clean up the rest of the sound to some degree.
Thanks for your input.
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some low level hum.
60hz or 120hz?
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60
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Does the PT heater winding have a center tap? If so, is it connected to chassis ground? If not, make an artificial center tap using two 100Ω 1/2W resistors and connect the junction of the resistors to chassis ground.
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^^^^what he said
guessing it's a SE PA?
if so;
gator-clip a big-ish E-cap across the cathode R, ~~~47uF
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The guy who worked on it before, and did a lot of work that looks pretty good, replaced the PT with a toroidal transformer, and replaced what I think is called a Field Coil Speaker "output transformer" with a standard 8 ohm OT.
Here's the schematic if you're interested.
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so no fil CT and cathode biased PP
artificial Fil CT and a big cap on the 200 ohm'r n the hum should drop
The neon on the heaters might be a cause for hum'n, not sure though
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Show us the tube side of the chassis. Please, no more postage size pics.
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I'm trying to send some photos, but having problems with the size.
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Don't know why it won't upload. RK101.jpeg
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When you open the attachment window read what's written below it. There's a filesize limit. A single file cannot exceed 2048KB (that's 2Meg).
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I know. It's 1.8M
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See if this one works.
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Here's #2.
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And the last, #3.
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> Here's the schematic...
Shows two 6N7. One of your bottles is clearly a 6SC7, and another may not be a 6N7 either?? A 6SC7 in the phase inverter with 6N7 resistors will be very unbalanced.
Have tubes been changed? One bad tube spoils the pot. We had someone was going to re-build his Twin but finally noticed that one 6L6 was just dead (so, not as much as a good Champ).
30% tolerance in pot makes little difference. (New pots may be almost as far off.) Volume controls vary, that's how they work. Same for the tone control which may be less useful than we expect today (more like a static-cut for long-range AM radio). All small resistors can be more than 20% out no big deal. Caps too, but 50% out starts to change things.
Shooter may be seeing a different schematic. I see CT and no 'neon' on heater-- that's a curlicue filament lamp like a Fender.
That parts-replacement style makes me shudder- that's how I would do it, very un-original. I suppose the amplifier can't see its own guts but the people around it might believe it better if it looked like Old Work.
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I see CT and no 'neon' on heater
i see the same thing......now :BangHead:
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Here is the symbol for a neon lamp, along with the symbol for a cold-cathode gas regulator tube. These are, in fact, very similar devices.
Note that each symbol incorporates a solid "dot" within the envelope, signifying the presence of a gas fill. You won't see this element on any standard incandescent lamp symbol, as they are low-vacuum devices.
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As for the tubes:
2- 6J7's
1- 6SC7
1- 6SC7 MG
2-6L6 GC's
1- 5V4G
I don't know when they were last changed.
I think the Fuse/Power Light holder was changed when the 3 prong lead was installed. The other 2 bulbs are incandescent to illuminate the pots.
All the caps and about 1/2 of all the resistors were already changed.
My replacement of the tone pot really cleaned it up a lot. Now just some low level hum.
The Mic Volume pot and circuit has a lot of gain as expected.
The Guitar jacks and circuit all seem to have a normal level of gain.
So it may be that this is as good as it's going to get. At the end of the day, the only thing "original" about this amp will be the chassis and the cabinet!
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Would you check the bias on this amp like you would on any other cathode biased amp, or is there anything wonky about this circuit that would affect the bias or biasing procedure?
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yep, voltage drop across the PA cathode resistor divided by the resistor value, the divided again since it's shared between the 2 PA tubes = tube current.
the PA B+ you should be able to get at The tap created when the field coil was replaced. at the PA tube gets sketchy if you flinch, twitch and short pins
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At the end of the day, the only thing "original" about this amp will be the chassis and the cabinet!
Was this amplifier ever owned by a guy named Theseus? He also owned a ship that you might have heard of. (https://en.m.wikipedia.org/wiki/Ship_of_Theseus)
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From my days of studying Latin and mythology, I do remember the guy Theseus. He had problems with minotaurs as well as ships!! Lucky he wasn't fooling around with tube amps!!!
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Latin and mythology
I wouldn't want someone speaking a dead language and telling tall tails working on amps either :icon_biggrin:
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Well I found the label on the toroidal transformer and it's a Antek AS 2T350. Attached is the spec sheet. Doesn't appear to have a CT. So, sounds like putting in the 100 ohm resistors might be a solution, at least in part, for the hum. Unless the other guy working on this did it in some other way. But I haven't seen any 100 ohm resistors on the heaters anywhere.
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just follow the wires, the wires tell all. check each socket, fil pins, usually a convenient place to add R's
verify both windings are used, or not, make note to self
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just follow the wires, the wires tell all. check each socket, fil pins, usually a convenient place to add R's
verify both windings are used, or not, make note to self
You can also measure resistance from a 6.3V filament pin to ground. If it's 50-100R, you have the resistors (or a pot) somewhere. If it measures open, then not.
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I've been kind of watching this project. I'm still wondering what this amp was originally built to amplify---lap steel--PA amp --or what? :dontknow:
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It's got 3 inputs in one channel for guitar. Then a separate channel that's hi impedance for a microphone.
It was a National Dobro schematic and the amp was made by Supro/Valco.
Maybe it was for harmonica vs. vocal on the mic.
I can't imagine 3 guitars playing into 1 amp.
That makes me ask the question "Why do amps, like Fenders et al, have a channel that is clean with no effects, and then another channel for Tremolo/Vibrato/Reverb?
If you don't want the trem/vibrato or reverb, just turn it off.
What's the reasoning? I'm sure someone out here knows!
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I hooked up my dim bulb current limiter to the amp just to see if there were any shorts/problems, etc. I had not done it previously and it was played with no problems, and I haven't made any changes to any of the components other than replacing the tone pot. I also hooked up my signal generator for input and connected the output transformer to a speaker. I plugged in the amp into the current limiter, and turned it and the signal generator on. The bulb briefly lit at a low level and went out, then as the amp warmed up the tone signal came through clearly, but also the light bulb started illuminating. It's a 150 Watt bulb. Wasn't at full brightness for a bulb of that wattage, but I'd say maybe 50%.
Is this a sign of trouble? I thought the bulb should only briefly light up and then go out.
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Well as an ole timer I can testify about two, three or more plugging into the same amp. Not every picker had a amp in the 40's, 50's and even into the 60's. Just for one instance I recall playing for a dance and our main sound system was my Gretsch 6164 Variety. It had a bass channel and normal channel. The bass, rhythm, lead guitar and vocal mike was all plugged into this amp. We had an un-miked drummer. Yes I understand the need for all those inputs perzactly!
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That's great! At almost 75, I guess I'm too young.
I've got the Gretsch Variety Plus 6165 amp that I got when I was in high school, around 1965-1966. I've also got a Gretsch Country Gentleman that I got about the same time. I've replaced 1 filter cap, and that's it. Still going strong. Plus it's got true vibrato. It has the normal and bass channel like yours too.
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but I'd say maybe 50%.
Would you check the bias
voltage drop across the PA cathode resistor divided by the resistor value
keep the bulb in-circuit, measure then post the current
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...I can't imagine 3 guitars playing into 1 amp....
In 1939, that's how they did it. Maybe in film-clips you see an amp per player, but they only made films of "good" bands, not working-class bands. And the Depression was still going on.
FWIW (maybe not much), in 1960s the Beach Boys had Sunn(?) or Fender(?) build one amp for the whole band, and vocals too? They did any gig they could get and needed to keep things simple.
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Cathode resistor is 229.3 and the Cathode voltage is 15.22 VDC so the bias is 6.63 ma. Pretty low?
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Sorry, or is it 66.3 ma?
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> or is it 66.3 ma?
Don't use long fancy numbers. Is 15/230 any easier?
When a problem exceeds my mind, I use tools. Slide-rule won't help you on this point, but any 4-banger or even the Windows Calc will do it.
(Ugh. WinCALC "Digit grouping" only groups to the left, not the right of the decimal point.)
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Ok 65ma it is.
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Sorry, or is it 66.3 ma?
Correct.
If you're dividing Volts by Ohms, the answer will be in Amps. Multiply Amps by 1000 to get mA.
0.0663A x 1000 = 66.3 (mA)
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now that we're all happy with 66mA, it's a shared cathode R so each tube is at 33mA, acceptable in my world for 6L6
take the bulb out of circuit and repeat measurement.
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So with the amp plugged directly into the mains, same input signal as before, same cathode resistance of 229 ohms, the cathode voltage is reading 27.4 VDC. So the bias is 104.8 ma.
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or 55ma a tube
what's the data sheet show for PP cathode biased 6L6 specs?
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WimWalther-I checked the resistance between ground and a filament pin, and its reading 279 ohms. The filament voltage is 6.7 vDC.
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ok, i looked it up myself
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Shooter-For two 6L6GC tubes, AB Cathode bias, the specs show 88ma for zero-signal plate current, and 132ms for maximum-signal plate current.
I don't know if that's the info you want.
I also used Rob Robinette's calculator. For my 6L6GC tubes and using my actual plate to cathode voltage of 379 vDC, his calculator shows 79.2 ma maximum safe dissipation for Cathode bias. And using my numbers and the cathode voltage drop calculator is shows Plate dissipation per tube of 47.7%.
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so now play it like you stole it :icon_biggrin:
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So Shooter, if there don’t appear to be any shorts and the circuit is ok, then why does the current limiter bulb illuminate and stay illuminated?
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100 mA at the tubes will light the bulb, how much, don't know.
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It seems like the bias voltage is higher than typical, and maybe that's just the way this amp works.
But I checked the DC rectified voltage at the first filter cap coming off the rectifier tube and it is reading 423 VDC.
That test point also has the wire connecting to the input of the Output Transformer. Is this input voltage higher than normal for an 8 ohm OT?
Since the schematic has no voltages, I have no way of knowing, but you guys might. If it is high, maybe that's what's affecting the bias, and it should be lowered.
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I also used Rob Robinette's calculator. For my 6L6GC tubes and using my actual plate to cathode voltage of 379 vDC, his calculator shows 79.2 ma maximum safe dissipation for Cathode bias. And using my numbers and the cathode voltage drop calculator is shows Plate dissipation per tube of 47.7%.
48% plate dissipation is completely fine in a PP self-biased PA
play it for 30 minutes, power down, let cool. bring it back up measure and re-calculate
if your meter has a Min/Max/Avg feature, clip it across the cathode Resistor, then play for 10 minutes. use the average value and re-calculate.
to "lower bias" you can bump up the 229 ohms to 270, 330 ohms. eventually you'll wind up in a catch-22 loop where B+ goes up as current goes down and the math stays the same. Then you have to lower B+, re-do the cathode R, repeat, it's fun, trust me :icon_biggrin:
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if there don’t appear to be any shorts and the circuit is ok, then why does the current limiter bulb illuminate and stay illuminated?
The brightness depends on the wattage of the bulb. Lower wattage bulbs will show some amount of reduced brightness, while a 250W bulb may be so dim you cannot even see it. If your bulb is as bright as it would be if screwed into a household light socket, then you would have a direct short on the primary side of the PT. If the bulb is considerably dimmer, you are probably fine. So, what is the wattage of your bulb and just how bright is it?
BTW, I have a pushbutton switch on my lamp bulb limiter so I can quickly apply full line voltage to the bulb to see just how bright the bulb is. I find this pushbutton to be blindingly useful. :laugh:
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A couple important notes when checking bias...
1. Bias is checked at idle so don't connect any input signal.
2. Amp must be plugged directly into the wall. (But if you have a variac I suggest now is a good time to use it. Plug the amp into the variac and adjust the line voltage to 120VAC.)
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Sluckey-Its a 150 watt incadescent bulb. Its low level brightness, still an orangish glow. So certainly not full out. It looks like this bias is going to be fine and doesn't appear to have any shorts. So I think I'll give this horse a rest until I can get the replacement pots and resistors installed and see what its like then.
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A 150W lamp running on 120VAC passes around 1250mA. A typical 6L6GC amp with a tube rectifier and 2-4 12AX7s will idle at under 1A, maybe 800mA.. that's enough for a strong glow, but well below full brightness.
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Can someone tell me what this resistor rating is? It reads 42K ohms on my meter, but the colors don't look right and I don't see a 42K ohm resistor on the schematic. There is a 4200 ohm resistor but this is reading 42K. Thanks.
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The significant digits appear to be "420", (yellow-red-black) but that's about as far as I get.
It could be "40" (yellow-black) + "00" (red) for a value of 4K. Might also be "42" (yellow-red) + "0" (black) for 420R. Just semi-educated guesses.
Btw, "value" designates resistance, whereas "rating" is power (wattage).
(ETA: Are we sure that's actually a resistor, and not just a really leaky cap?) ;-)
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Thanks for your input. Yea, I should have chosen my words better. Looks like Silver is the last band for the "rating".
I've never seen a cap like this. It doesn't read anything when my meter is set to cap. So, it's got to be a resistor. The color bands are what's confusing me. I don't know if Yellow is just a background color, or it's part of the code. I've got another one that's similar in size and appearance that has a Green "background" that reads 500K and there's no 500K resistor on the schematic.
And this is an original resistor, so it's been there since 1939. So the schematic seems to indicate it could be a 20K based on its location in the circuit, but I don't know unless its serving as a resistor for two 20K resistors on the circuit.
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Can someone tell me what this resistor rating is? It reads 42K ohms on my meter, but the colors don't look right and I don't see a 42K ohm resistor on the schematic. There is a 4200 ohm resistor but this is reading 42K. Thanks.
A huge clue would be what it actually connected to in the amp. Then you could look at the schematic and know what it should be.
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I did that, and its connected at one end to the jumped Plates at pins 2 and 5 on the first 6SC7 tube, with the other side going to a terminal type connection on the socket housing (not a lug) of the second 6J7 tube where several wires are mounted. The only resistor coming off those jumped Plates is a 20K resistor that heads over to the 6SC7. So although I think I know the connection from the schematic, the value shown in the schematic is way different from the actual reading.
But I can't figure out what the value of this old resistor is. Is it a 20K? I don't know how to read an old resistor like that. So I don't know if it's a bad 20K resistor, or if a larger resistor was substituted when the amp was built. There's another one, similar in design but different colors, that reads a little over 500K, but the circuit is showing a 250K.
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I did that
and the schematic we're looking at doesn't line up with tubes you have
are you SURE it's not a coupling cap from the plates? did you find a coupling cap at the same point?
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I did that, and its connected at one end to the jumped Plates at pins 2 and 5 on the first 6SC7 tube
Ain't no 6SC7 tube on the schematic you posted. You really need to confirm which type tubes should be in this amp before proceeding. A 6SC7 and 6N7 are not pin compatible.
I don't know how to read an old resistor like that.
Read this... (https://www.radioremembered.org/rescode.htm) Your resistor is shown in figure 2. The code is body, end, band. So your resistor is yellow, black, red (or orange). Value would be 4K (if red)or 40K if orange.
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But I can't figure out what the value of this old resistor is. Is it a 20K? I don't know how to read an old resistor like that. So I don't know if it's a bad 20K resistor, or if a larger resistor was substituted when the amp was built. There's another one, similar in design but different colors, that reads a little over 500K, but the circuit is showing a 250K.
The other resistor is green black and yellow. If you were to read it as BED, (Body, End, Dot) it would be a 500k resistor, which is what it reads on your meter. Which would make this one a 4k resistor, (Yellow, Black, Red). Are you sure you measured 42k, and not 4200 ohms?
I'm not saying that's what it is for sure, but it kind of makes sense judging by the other resistor and your measurement.
Edit: after reading sluckey's response, I suppose it could be orange?
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The tube in the 3rd position is a 6SC7 and the one in the 4th position is a 6SC7MG.
The color I'm pretty sure is orange, so that's 40K, and my meter is reading the resistor, out of circuit, at 42.8k.
As to the other resistor, its colors, based on this chart, says its 500K, and it reads a little over that number.
But there are no such resistors shown on the circuit's schematic that I have and that I have posted to this site. This amp may have been built on a different schematic design but I don't know where I'd find another schematic.
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I suggest you quit trying to match that schematic to this amp. They clearly don't belong together and will only lead to heartbreak if you continue.
This pic shows the colors better...
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I think you're right about that.
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Thought you may want to see this...
&ab_channel=UncleDoug
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The tube in the 3rd position is a 6SC7 and the one in the 4th position is a 6SC7MG.
But are they right??? Lots of amps get stuffed with any old tube. Any idiot can do that. Did that amp ever work for you with this tubeset?
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I’ve watched that video before and during this episode, and actually watched it again today and confirmed that his amp and mine are the same, the tubes are the same as mine, and the questionable resistors are the same as mine. So I’m satisfied that my amp changed from the schematic I got.
This amp worked with these tubes, it just had a lot of noise and scratches and hum. I think most of that is pot related, which I have replaced. So will probably light it up tomorrow and see how it sounds.
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Lot's of info on this amp on the net... Everything except a correct schematic. The correct tube line up is 6J7, 6J7, 6SC7, 6SC7, 6L6, 6L6, 5V4.
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I’ve watched that video before and during this episode, and actually watched it again today and confirmed that his amp and mine are the same, the tubes are the same as mine, and the questionable resistors are the same as mine. So I’m satisfied that my amp changed from the schematic I got.
This amp worked with these tubes, it just had a lot of noise and scratches and hum. I think most of that is pot related, which I have replaced. So will probably light it up tomorrow and see how it sounds.
Yeah, light it up, through a limiter first of course. The main issue, if any, is that there was somebody else in there already. Did they replace parts from what they saw, or the schematic, (which isn't accurate)?
If it works, fine. If not, you may have to draw out the circuit you have, and test voltages at every pin to figure out what is actually going on.
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The color I'm pretty sure is orange, so that's 40K, and my meter is reading the resistor, out of circuit, at 42.8k.
There you have it. The band comes across as very much red on my screen, but orange makes better sense.
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This pic shows the colors better...
It sure does.. the band I'd identified as red is clearly orange in this pic.
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Got it finished. I replaced that resistor and 4 or 5 others that were way out of spec, replaced the 3 pots, and the resistors for the input jacks. Sounds really good and pretty clean. It can get loud. No noise until its turned all the way up. No hum that I can tell. I didn't install the 100 ohm resistors.
Thanks for everyone's help on this project. I doubt I'll ever see anything like this one again. It's been fun and as always a learning experience for me. So that's good. Best.
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Before I return it, I noticed this morning that when the amp was turned on and just got warmed up, the rectifier tube "popped" once with a flash, and then settled down and was quiet. I ran it for a little while, then turned it off for an hour or so, and then turned it on. This time no pops, flashes, noises, etc. Sounds fine.
I also tapped the tubes and there seems to be a lot of resonance on the 1st 6SC7 tube, and some on the 2nd 6L6GC tube. Nothing from the rectifier.
Any thoughts on this?
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Tell your friend that the tubes are old and tired and he should buy a complete spare set.
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That was my thought too.
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I guess "microphonics" is the better word.
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I have a question about the sizing of the Output Transformer in this amp which is running two 6L6GC tubes and is cathode biased.
The OT was installed by someone else to change to a modern design using an 8 ohm speaker. I can't find any label on the OT. It has the standard 2 leads for the Primary, and a red center tap lead, and a black and blue lead on the Secondary.
I watched some videos on determining the Impedence Ratio of OT's, and did the following: After unplugging the amp and removing the Output Tubes, I put a 10 VAC on the black and blue leads, and read 480 Volts AC on the Primary leads. I determined the Votage Ratio is 48:1. Then to determine the Impedence Ratio, I squared 48 resulting in an Impendence Ratio of approximately 2300:1.
Ok, if that is correct, and based on the tube chart for the 6L6GC's they have Effective Load Resistance Plate to Plate rating of either 6600, 3800, or 5600 ohms. I don't know which one to use, or should I be looking at some other spec?
But with this information, can I determine what the ratings on the OT should be when I'm using a 4 ohm or a 8 ohm speaker?
If so, how do I do it?
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But with this information, can I determine what the ratings on the OT should be when I'm using a 4 ohm or a 8 ohm speaker?
If so, how do I do it?
You say the impedance ratio is 2300:1. Same as 9200:4, or 18.4K:8. I would not use that OT. My choice is Hammond 1760J (https://www.hammfg.com/files/parts/pdf/1760J.pdf?t=1697661946).
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Is that because its too big, or some other reason?
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Is that because its too big, or some other reason?
The impedance ratio is too big. Not optimum for 6L6s.
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Thanks Sluckey for your help on this.
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I don't know which one to use, or should I be looking at some other spec?
typically there's 3 columns, the top line is typically plate VDC.
I use as many "close enough" of these values that match what I "expect" for my particular amp to narrow which column I use.
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Shooter-I looked at the spec sheet for the Hammond OT that Sluckey posted, and it says it's 40 watts.
I'm trying to learn more about OT's, and wanted to learn how to calculate the wattage output of an OT. I'm curious as to what it is for the existing OT that I've got in this amp. Can you help?
Also, what determines the wattage of a speaker that you want to use with a particular amp? For example, if you've got a 20 watt amp, what size, ie. watts, speaker should you use. I've seen speakers, say at Weber, that you can buy in 2 different wattages. What should one consider in matching a speaker with an amp, or for that matter the amp's OT?
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how to calculate the wattage output of an OT.
the simplest method is the data sheet from the OT maker.
an OT is simply a step-down transformer, so the input side will have many a wind of smaller copper, the output has less windings with bigger copper
you can do some crazy math on wire diameter, current ratings of wire diameter over time
so in your case of "unknown OT" in a working amp
work it hard for 15 minutes, get slightly warm, quite warm, hot?
if 1 or 2, work it another 15 minutes, retest hotness.
a proper OT worked hard for an hour will be warm to quite warm but not "to hot to touch"
speakers are one of those "mojo" math things, some guitarists want an under-rated speaker that will distort as part of their "sound"
others want over-rated for fidelity, longevity, etal.
pretty much everyone wants ones that "match" ~~~close to designed wattage of amp, AND have the "sound" they're looking for.
I rolled about 12 speakers through a 2X12 cab til I found 2 that worked well together for "my sound", both were ~~ 40W speakers using a 23W rms amp. This covers guitar amps, Audiophile speakers and usage is a completely different set of Mojo-math
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here's the data sheet for the PA tubes in my last build
I selected the parameters i would "shoot" for from the data sheet (red box)
build n tweaked amp to both sound good and stay "close" to the datasheet values
notated the datasheet with final measurements
before all that, I knew what PA tubes I wanted by the datasheet "default" values to buy the PT OT. you'll notice the datasheet "wants" 12k plate-plate. the one I bought was 10K
part of my "tweaking" was to adjust plate VDC, cathode bias (Vk), for "biggest" clean signal at speaker
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So am I correct that the Plate Voltage of the OT's will affect the Output Volume of the amp, and the Output Volume is not affected by the Wattage of the OT? And if that is the case, then the Wattage of the OT could not harm either the Output Tubes or the Speaker? In essence you want to select a OT that can handle enough Wattage that the Output Tubes produce?
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Audio transformers never over-heat. The iron and copper to reproduce bass cleanly is the "power limit". Tastes vary about what is bass and how much distortion to tolerate. But a 100W amp on a 10W transformer may flub-out playing bass runs.
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Thanks for your input.