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 (http://music-electronics-forum.com/s...ead.php?t=8409) 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)]

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]

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)]

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]

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...