Power tranny gets HOT in old Guild amp

[slidincharlie (Carlo P)]

The '60 Guild 99-J amp that was the subject of a recent thread of mine (Is this circuit ok or has it been hacked? Please advise (vintage tube amp)) is working and sounds good through a new speaker (original Jensen broken).

Trouble is that the power tranny gets fairly HOT, and after about 30-40 minutes operation I can't barely grab it for more than 6-7 seconds. Too hot!
None of the power tranny in any of the amps I've ever played or serviced has got that hot.

What can cause such heating? What should I check/measure?

TIA,
Carlo

[Steve Conner]

Could be the 50/60Hz thing. If they skimped on iron in the transformer, 50Hz will make it run hotter. You could try lowering the voltage to 100V (5/6 of 120V)

[MWJB]

A lot of old, budget amps were built with little or no headroom on PT current capacity, subsequently they can run too hot to touch at all, even for as long as 6-7 seconds! Often, they have run like this for years.

I see that this amp has a replacement PT, it's possible that the guy who installed it didn't correctly anticipate heater current draw, or B+ secondary current draw (a 210ohm cathode resistor with 6L6s running over 350vdc on the plates will cause a big draw on the PT). Without more info it is impossible to say whether the new PT is any better or worse than the original.

What are your heater & power tube plate voltages? Are there any identifying no's on the PT? Does rectified B+ voltage equate to what you might expect from the secondary AC winding?

You can try one or either of these:

1. Remove the tremolo 12AX7, you'll lose the effect of course. This will free up 0.3A of heater curent, it won't make a huge difference overall but may give you a little more room to move after dropping B+ current draw. Obviously, if you like the sound of the tremolo, this is not an option.

2. Try rebiasing the 6L6s to run a little cooler, with that 210ohm cathode resistor you probably have scope to get somewhat cooler without ruining the tone of the amp, especially if plate voltage (to ground) is higher than 350vdc?

You might possibly find that heater voltages are under 6.3VAC and that biasing cooler will bring heater voltages back into range?

In some, rare cases a PT's primary winding may have some shorted turns, this makes the PT run hotter than normal and pushes up all secondary voltages - I wouldn't necessarily jump to this conclusion in your case, not without much more specific info.

[MWJB]

"- a replacement power tranny with 230V primary (this amp comes from the UK);" What's your voltage from the wall? You might temper things by trying the 250v taps if you have them?

[slidincharlie (Carlo P)]

Steve,
I'm not sure to understand what you mean.
If you refer to the mains voltage, I live in Italy where we have 230V from the mains, so there should not be any 50/60Hz issue with a 230V power tranny, right?

MWJB,

first of all for some reason the power tranny today was way less hot than yesterday...

I took a few readings:
5.8VAC across the heaters instead of 6.3
285 VAC to the ss rectifier
333 VDC on each 6L6 plate
22VDC across the 210 ohm (actually 206) cathode resistor.
This makes for around 50 mA idle current for each 6L6!
I suppose I must have those tubes run cooler (with a larger cathode resistor) in any case, right?

I put here a photo and some specs of the power tranny, if this gives you some clues.
Codes: S-200 682017. 682=Electrical Windings EIA code?
Secondaries: two 6.3VAC 2A center tapped, one 6.3VAC non-center tapped secondary, and one 300VDC center tapped .
The heater wires are connected to one of the 2A filament secondaries.

[MWJB]

You've got a 2.7A current draw on your 2A 6.3VAC winding! No wonder it's running hot & your heater voltages are low.

2 options:

Change the PT (if you can find one that will physically fit, then rebias accordingly).

Change the 6L6s for 6V6s and correct speaker impedance accordingly. 6V6s are better suited to an amp with a 285-0-285 secondary, will probably be louder than 6L6s in this amp.

[slidincharlie (Carlo P)]

Originally Posted by MWJB

2 options:
Change the PT (if you can find one that will physically fit, then rebias accordingly).
Change the 6L6s for 6V6s and correct speaker impedance accordingly. 6V6s are better suited to an amp with a 285-0-285 secondary, will probably be louder than 6L6s in this amp.

The second option looks more feasible.
May I keep the current power and output trannies if I switch to 6V6?

[MWJB]

Yes, the PT should be fine with 6V6s, the OT will too but you should establish the turns ratio/primary Z (vintage 6V6 amps may have had a primary Z anywhere from 4.5K to 8.5K), or use a speaker with twice the impedance rating compared to what you used with 6L6s.

[Robert M. Martinelli]

Originally Posted by slidincharlie (Carlo P)

Steve,

Secondaries: two 6.3VAC 2A center tapped, one 6.3VAC non-center tapped secondary, and one 300VDC center tapped .
The heater wires are connected to one of the 2A filament secondaries.

Hi,
It could be that your PT was intended for amps with a tube rectifier, so one of the 6,3 VAC secondary windings is center tapped ( for hum balancing ) and one is not ( the one intended for the rectifier ), the problem IMHO could be that one of the secondaries could be 5 VAC ( for rectifiers like 5U4, 5AR4 and so on which have 5 V heaters ) instead of 6,3VAC.

If this is the case you could be wired to the 5 VAC sec and maybe, having a higher than normal mains voltage, you' re getting 5,8 VAC there. I live in Italy too, and I encourage you to check your mains voltage from time to time, to see if it' s higher than the expected 220 VAC and if it varies ( depending on the load, so probably higher in the late evening when people' s asleep ) in different moments of the day. Remember a 10% voltage increase causes a power increase of about 20%. A friend of mine successfully sued the electric co. because his mains voltage was 265 VAC instead of 220 VAC, so the power dissipation was 50% over the specs and this caused several appliances ( TV, VCR etc. ) to fail.

If both the secondary windings are indeed 6,3 VAC and you really have a low voltage on one caused by excessive load, I see no reason you should not use both 6,3 secondaries to give relief to the one currently used, even if the other secondary is not center tapped you can always add a hum balance pot ( well, you could do the same also on the first one, but you should disconnect the center tap first ).

Hope this helps

Best regards

Bob

[Steve Conner]

You said that you have two 6.3V 2A windings, but you're only using one of them. Why not parallel them, or just connect half of the tubes to each winding.

About the 50/60Hz thing, I assumed you were using an imported American amp with a transformer, I didn't realise the PT had been replaced for a Euro one.

[MWJB]

Is there a current rating for the 300v tap?

[slidincharlie (Carlo P)]

Originally Posted by MWJB

Is there a current rating for the 300v tap?

There is a "150MA" marking close to the 300V taps, but it's more in between the used 6.3V taps and the 300V taps. I don't know what does this refer to.

[MWJB]

Most likely it's B+ current rating for the 300v taps, just about enough to run 6L6s in cathode bias (if it had been 100-120mA, 6V6 would really be the only option - still are a good alternative) if you follow Steve's suggestion re 6.3v taps. You'll probably want to rebias once the heaters & B+ have risen to normal levels.

[Robert M. Martinelli]

Originally Posted by slidincharlie (Carlo P)

There is a "150MA" marking close to the 300V taps, but it's more in between the used 6.3V taps and the 300V taps. I don't know what does this refer to.

It refers almost certainly to the 300 V tap, 150 mA looks reasonable as a plate supply current.

[slidincharlie (Carlo P)]

1) correction: the current marking between the used 6.3V taps and the 300V taps is 120MA, not 150MA.

2) the unused 6.3V taps (the center tapped and the non-center tapped) read both a solid 6.38Vac (now the mains supply is 238Vac). Thus I could simply switch from one 6.3V (actually 5.8) 2A secondary to another. Trouble is that the center tap in the unused 6.3V secondary is broken. May I connect the filament wires to this secondary and reference them to ground via two 100 ohm resistors wired on the pilot lamp, a la Fender? In this case I wouldn't need a center tap, right?

3) for some reason the PT today is cooler, but the 6L6s are VERY hot after only 3-4 minutes operation...

[Marc]

Carlo,
In the picture earlier in this thread there is what appears to be an older paper jacketed electrolytic, is it possible that there are others that may not have been replaced and are passing curent?

I have two of these amps in my shop now with original trannies, both slightly different circuits, but neither have hot PT's....

Marc

[slidincharlie (Carlo P)]

Marc,
that's the cathode bypass cap on the 6L6's. I haven't replaced it yet because I don't have a replacement now, but I've changed all other e-lytics.

[slidincharlie (Carlo P)]

There is another thing that I've noted in this amp: the power tranny "vibrates" when the amp is on. It'a a sort of weak high frequency vibration that I can easily detect if I touch the tranny or even the chassis. Maybe it's normal, but I've never noted such kind of vibration in other amps before.

[MWJB]

Not necesarily abnormal if it's a light "tingling", if the PT is buzzing loudly it's a problem.

[MWJB]

With a 120mA B+ I wouldn't run 2x6L6 & 3x12AX7 if I had the choice.

If you rewire the 6.3 supplies to bring up the heaters, you will also bring up the B+, those 6L6s will draw 60 to 70mA-ish in that circuit (by the way, the reason your 6L6s are hot after a few minutes is because you are running high idle current in cathode buias - it's normal), if you rebias B+ voltage will rise again.

With that PT I'd run the 6V6s (& check/correct impedances if necessary), in fact even if you ran all 3x12AX7 AND the 6V6s together on the same 2A winding the PT would be under no more stress than if you just ran the 6L6s alone to one 2A winding.

[slidincharlie (Carlo P)]

Originally Posted by MWJB

Not necesarily abnormal if it's a light "tingling", if the PT is buzzing loudly it's a problem.

No buzz. It's like when you have a mechanic thing going on... think of a small electric fan held in your hand... or your fridge when you put a hand on it, near the back... a silent weak vibration that transmits to the whole chassis.

[MWJB]

Not particularly abnormal. Check a few other amps, I'm sure you'll find others that do the same.

[Steve Conner]

That kind of vibrating feeling often means that current is leaking to the chassis. It's basically a minuscule electric shock. Some people don't feel it as a vibration, they say that the metal surface feels furry or rough.

It's to be expected with Class II double insulated gear, which is allowed to leak a little through stray capacitances, but if you ever feel it on any metal that should be grounded, it's cause for concern, because it suggests the ground connection is bad.

[Robert M. Martinelli]

Have you checked the tranny' s bolts to see if they 're a bit loose? If this is the case then the whole thing is going to vibrate at mains frequency...and a side effect of having the bolts and the iron pack loose is a loss in the tranny' s efficiency, so it' s temperature could rise to some degree...

[R.G.]

If you can't also HEAR the chassis, only feel the vibration, then Steve is right - you have AC leakage.

You need to test this by putting a voltmeter set to AC Volts between the chassis and the third-wire safety ground on the AC supply. If there is more than 1-2Vac, then you are leaking AC power into the chassis, and the amp is not electrically safe to use.

As to the original heating problem: if the amp had old filter caps, they could have been leaky and overloading the transformer. In time they may have re-formed a bit and now be less loading. In any case, replace the filter caps unless you have already put new ones in.

It is also possible that high voltage is leaking to chassis through the power supply choke.

It is also possible that there is an internal shorted winding to the core of the transformer.

Do the chassis AC voltage test first, and replace the transformer if you have AC leakage. In any case, change it to 3-wire safety ground on the chassis for safety reasons.

[slidincharlie (Carlo P)]

I'll check and clean all the ground points, because there is some rust and a sort of greasy residue all over the place.
The bolts are firmly secured to the chassis, so this should not be the cause of the vibration...

[Robert M. Martinelli]

Ooops..

Sorry folks, I mistakenly thought the vibration was mechanical, and consequently gave a wrong advice - didn' t mean to mislead Carlo - beg your pardon

Bob

[slidincharlie (Carlo P)]

Steve described well this particular "vibration" in my amp: "That kind of vibrating feeling often means that current is leaking to the chassis. It's basically a minuscule electric shock. Some people don't feel it as a vibration, they say that the metal surface feels furry or rough."
The vibrations starts as an audible rattle as soon as I turn on the amp, then it remains as a sort of minuscule electrical shock.
There is no AC on the chassis though.
BTW, filter caps had already been replaced and grounded power cord added.
I've also noted that the tranny gets hot randomly; sometimes it stays perfectly cool. Maybe there's an intermittent internal short in one of the windings?
I'll check the ground connection, then switch to the unused 6.3V 2A heater secondary.
I'll post my results at the end of the job.
Thanks!

[Steve Conner]

Hmm, maybe the power tranny has an intermittent short to the core? Have you tried running the amp through a RCD plug? I don't know if you have them in Italy, but they're available in electrical stores here.

http://www.chsinteractive.co.uk/elec...mpaign=froogle

If anything was wrong, one of those would trip out.

I always get antsy when I feel that feeling and start checking ground wires and the like. Sure, it could just be mechanical vibrations from the transformer, but then again it could mean a bad ground connection and an accident waiting to happen.

This reminds me of one time I was sitting on the couch playing guitar after hanging some pictures. It turned out that I'd put a nail through the cable feeding the outlet that my amp was plugged into, severing the ground wire and shorting it to the live. I don't remember if I felt the furry feeling, but I did notice my amp was humming a lot more than usual. I touched the guitar strings against a central heating radiator to see if I had a grounding problem, and the lights went out.

[slidincharlie (Carlo P)]

I have switched to the unused 6.3V 2A center-tapped secondary (which reads about 6.3V unloaded) (top left in the attached sketch), but it reads 5.8-5.9 VAC under load like the previous one! Thus there must be something in the circuit that causes this.
Any idea?
(haven't tried a pair of 6V6 yet).
I'd appreciate instructions about using the two 6.3V secondaries in parallel, like suggested above. In attachment please find a sketch of the PT.

[Robert M. Martinelli]

Hi Carlo,
It definitely seems your tranny' s heater voltage gets lowered by excessive load,
( to help us help you always remember to specify the measurements' conditions, open/loaded circuit, st-by on or off etc... ) so it' s time to do something about it.... Well - you have two options - The first one is to connect in parallel the two 6,3 VAC windings - If memory serves me well you told that one of the center taps was broken, from the sketch you posted I see the second center tap is connected to the power tubes' cathode. Make sure you have both the 6,3 VAC secondaries disconnected from the circuit, then switch on the power and measure the AC voltage on both of them - they should read very similar voltages ( around 6,4 - 6,6 VAC, remember no load is present at this time ) - If they don' t, the "parallel" option is not feasible. If they do, OK, you' ll simply have to connect them in parallel, ignoring the center taps ( leave both of them disconnected ), then check the phase, there is a chance you get 0 VAC when paralleling because the two windings are out of phase, don' t fret, simply reverse one, parallel them again and you should read 6,4-6,6 VAC again.

At this point you' ll have a 6,3VAC-4 Amps secondary, and you should not see the voltage dropping to 5,8 VAC when connecting it back to the heaters.
You might like to add a hum balance pot, just take a 100 to 220 Ohms potentiometer and connect its center lug to GND, and the other two lugs to the 6,3VAC, and adjust for minimum hum, which you should get around the pot' s center position ( IMPORTANT - when using a balance pot all center taps from the tranny MUST be unused, so double check ).Et voila' - the job is done!

Second option ( If you like this more, or if the parallel option is not feasible because the two 6,3 VAC secondaries have uneven voltages ) :

Leave the 6,3 VAC secondary you' re currently using ( the one with the center tap tied to the power tubes' cathode ) only for the power tubes, and power the preamp's heaters with the other 6,3 VAC secondary - This option allows you, if you like, to power the preamp tubes' heaters with DC rather than AC, ( the final stage, being a push pull, is a hum cancelling design, but the preamp is not ) - This way the preamps' tubes will be quieter - If you' d like to go this way let us know and we can deal with this later on.....

Hope this helps, and hope I managed to be clear enough...

Best regards

Bob

[MWJB]

"I have switched to the unused 6.3V 2A center-tapped secondary (which reads about 6.3V unloaded) (top left in the attached sketch), but it reads 5.8-5.9 VAC under load like the previous one! Thus there must be something in the circuit that causes this.
Any idea?
(haven't tried a pair of 6V6 yet)." So, hang on what have you done to fix the excessive load on the PT's heater winding? Your PT is greatly overstressed, there's little point in reporting the symptoms until you take some action to correct this.

It doesn't matter which winding you use, 2.7A into 2A doesn't go. You could run the 6L6s to one winding and the preamps to another but I wouldn't advise running 6L6s in cathode bias, with high plate current, with a 120mA B+ winding.

[Robert M. Martinelli]

Originally Posted by MWJB

"I have switched to the unused 6.3V 2A center-tapped secondary (which reads about 6.3V unloaded) (top left in the attached sketch), but it reads 5.8-5.9 VAC under load like the previous one! Thus there must be something in the circuit that causes this.
Any idea?
(haven't tried a pair of 6V6 yet)." So, hang on what have you done to fix the excessive load on the PT's heater winding? Your PT is greatly overstressed, there's little point in reporting the symptoms until you take some action to correct this.

It doesn't matter which winding you use, 2.7A into 2A doesn't go. You could run the 6L6s to one winding and the preamps to another but I wouldn't advise running 6L6s in cathode bias, with high plate current, with a 120mA B+ winding.

Hi MWJB,
Looks like we came to the same conclusion, the tranny' s 6,3 sec is overloaded; in my previous post I suggested Carlo either to parallel the two 6,3 VAC 2A windings to get 6,3 VAC - 4Amps, or to use one winding for the preamp section' s heaters and the other for the power tubes' heaters - the latter solution would also allow him to power the preamp' s heaters with DC if he would - which one do you like best ? Your opinion is highly appreciated.
Regards
Bob

[MWJB]

Hi Bob,

As far as the 6.3VAC supply goes, either the parallel option or running preamps & power tubes from separate 2A windings would be fine (personally i don't think it necessary to switch preamps to DC heaters), BUT even if Carlo does this and rebiases cooler with a 250-270ohm cathode resistor the 6L6 plates will still be drawing all the available B+ current...and then some, possibly to the extent that this will still pull down secondary voltages (inc. the heaters)?

Of course Carlo could rebias cooler still with 330ohms or so but there will be a noticable change in character.

This is why i suggest dumping the 6L6s with this particular PT. With a pair of 6V6s and all 12AX7 connected to one 6.3V 2A winding, the load will be no different to running just 2x6L6 to one winding. There will be adequate B+ current supply, Carlo should still need to sub the 210ohm cathode resistor for 250-270ohms as the B+ voltage will subsequently rise due to the 6V6s drawing less cuerrent than 6L6s.

With the 300-0-300VAC winding the amp will probably sound just as loud with 6V6s.

I really feel that to persevere with 6L6s a new PT is order. It really just amounts to what Carlo feels is the most practical option: A new PT, or sub out the power tubes and possibly change the speaker to bring impedances back into line?

[Robert M. Martinelli]

I also have a third choice, ( sorry Carlo don't want to confuse you )
Why not connect the 6L6s as triodes? This way the plate current would go down significantly ....together with the output power....

Using 6V6s would be a good solution though.... they should also have a little "edge" over 6L6 from a "sonic" point of view, the impedance mismatch would be no big deal to fix, as Carlo could use the same speaker(s) with the help of series and/or parallel power resistors to match it.

Both these options are good if Carlo likes the sound and doesn't care much about output power.

Carlo, it' s your turn! We' re looking forward to hearing from you soon that everything' s been fixed and, most importantly, that the thing sounds great to your ears/taste!

Regards

Bob