# Help wanted on EL34 and 12AX7 tubes

#### usalabs

Oct 21, 2016
10
I'm trying to repair a guitar amp head, and it uses 4xEL34 matched pair power tubes for a total output of 120W and 4x12AX7 pre-amps, now I've replaced all the tubes, and checked for bad solder joints, and even checked the power tubes bias, and heater voltages according to the schematic, and I still can't stop the power tubes from glowing a bright orange, and an increasing crackling sound from the speaker, the head is a Peavey 3120, I am unable to use an AF injector + oscilloscope as I can't keep the amp powered on to long, I don't want to explode the tubes, the longer it's powered on, the louder the crackling and brighter the tubes glow.

Visual inspections show no sign of burnt components, and quick testing of any diodes don't show anything open or short, resistors around the power tubes for bias, and input show no deviation from their resistance values, and a test of the output transformer shows no sign of shorting.

The output impedance is switchable from 4, 8 and 16 ohms, but the 2 outputs are parallel, and according to the manual, the cabinet speakers have to have a resistance load of 8 ohms min per speaker within the cabinet, with the amp switch to 4 ohms, but I later found out that the guitarist was using 2x4 ohm speakers, and using the equation for parallel resistance, the head should be switched to 2 ohms, but the head doesn't have a 2 ohm setting, and reverse parallel equation shows that if the head is switched to 4 ohm, then the cabinet speakers must be 8 ohms each.

The equation I used is:-

RTotal = 1/((1/R1)+(1/R2))
giving that R1 and R2 both equal 4ohms, resulting in RTotal = 2ohms

In theory the mismatched impedance between speaker load and head output could have done something to the power output stage(s), but I'm stuck as to what.

I have exhausted everything I can think of and test, maybe someone can think of something I haven't.

#### duke37

Jan 9, 2011
5,364
I do not know what an amp head is. Two countries separated by a common language?

You need to put a load on the output for safety, a large resistor is best. If necessary use a short. (Do not do this with a transistor amplifier, they can be tested with an open circuit load)
Short the input so there is no signal.
Run the amplifier through a lamp limiter to protect the valves (USA tubes).
Measure the voltages, they will be low because of the limiter but you should have an idea of where the fault my be.
If the voltages are reasonable, check for high frequency oscillation with an oscilloscope. Beware that many recent scopes do not like a few hundred volts shoved up them.

Show us the circuit.
Be very careful, we do not wish to lose you.

#### davenn

Moderator
Sep 5, 2009
14,296
In theory the mismatched impedance between speaker load and head output could have done something to the power output stage(s), but I'm stuck as to what.

I have exhausted everything I can think of and test, maybe someone can think of something I haven't.

what specifically is glowing bright orange ? .... the plates or the filaments ?

I would assume, like most tube amps, it has an output transformer for impedance matching, it's quite possible it has a shorted turn or 2. a very common fault in tube gear

and I also have no idea what you are referring to, re head amp ??

Dave

#### 73's de Edd

Aug 21, 2015
3,636
Sir usalabs . . . .

( Not Brittania-labs ? )

This sounds like a classic case of output tube grid mis-biasitis.
Let's use my markup test points for making some initial evaluations .
Have proper speakers… hooked up to the proper terminals on the amplifier and then use minimum volume setting,
Unless bringing up slightly, just to listen for the errant popping noise.
As soon as the tubes filaments warm up, take note of and initially log the negative voltage level being read between reference points B and C .
Expecting it to be at the negative 40 V range, but advance its control on up to max in order to see how much of a negative voltage swing is being possible. Log down that maximum voltage also .
Then bring it back down to a negative 42 volt setting.

My A reference is merely pointing out that the bias supply on this unit is being derived from voltage multipliers
with associated electrolytic capacitors. This old pup is getting on up into the 15 year age region . . . but . . .who am I to talk . . ..
See if those output tubes are not running cooler now.
If so confirmed, that will now let us move to some other points and take readings.

Main B+ reading from D needed, using K ground referencing . . .log.

Plate voltage readings of F and E needed, using K ground referencing . . .log.

Measure the voltage across each of the 100 ohm resistor at G-H-I-J and G'-H'-I'-J' . . .log.

Move back to the BC bias point and recheck the bias that was set ands see if there has been any voltage variance,
THEN you hold that reading and unplug that V4 12AX7 tube which feeds the output tubes.
Did that bias voltage then decline any ?.
IF there had ever been any random popping . . .did it stop when the 12AX7 was unplugged ?

Plug the 12AX7 back in.

Lastly do a smart finger thump to each output tube, to see if any internal flashover occurs.

Last info is acquired in no power condition, where you switch to ohmmeter mode and move back to D:
Measure the resistance from D to F . . . .log.
Measure the resistance from D to E . . . .log.

Techno Referencing:

73' de Edd

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#### duke37

Jan 9, 2011
5,364
Take out the EL34 valves and measure the voltages at the valve sockets. There needs to be significant negative voltage, look up what is required from valve data. There are capacitors from the 12AX7 which can give positive voltage if leaky.
Mullard gives
800V anode
400V screen
-39V grid
Power (push/pull pair) 100W

#### usalabs

Oct 21, 2016
10
what specifically is glowing bright orange ? .... the plates or the filaments ?

I would assume, like most tube amps, it has an output transformer for impedance matching, it's quite possible it has a shorted turn or 2. a very common fault in tube gear

and I also have no idea what you are referring to, re head amp ??

Dave

The plates glow orange. I'll explain the steps that lead to the orange glows.

1........turn on the standby switch
2........Power on the head (amp head is a description for the actual amp that uses separate cabinets for the speakers, whereas a combo would be the amp and speakers built into one unit)
3........Wait about 15 to 30s for quick warm-up
4.........switch off standby, and as soon as the standby switch is toggles to off, the 4 EL34's start a small blueish glow at the bottom near the heater filaments, then the plates start glowing orange and the longer the unit it powered on, the brighter the tubes glow and the louder the crackle, lowering the master and channel volumes have no affect, I did replace all the tubes though to make sure.

#### usalabs

Oct 21, 2016
10
I do not know what an amp head is. Two countries separated by a common language?

You need to put a load on the output for safety, a large resistor is best. If necessary use a short. (Do not do this with a transistor amplifier, they can be tested with an open circuit load)
Short the input so there is no signal.
Run the amplifier through a lamp limiter to protect the valves (USA tubes).
Measure the voltages, they will be low because of the limiter but you should have an idea of where the fault my be.
If the voltages are reasonable, check for high frequency oscillation with an oscilloscope. Beware that many recent scopes do not like a few hundred volts shoved up them.

Show us the circuit.
Be very careful, we do not wish to lose you.

73's de edd has the schematic in a lower post.

Oh there's no problem there with safety lol, I've had so many HT shocks from tube anodes, I would have thought I l learnt my lesson by now lol, I've even been thrown on my back from a static discharge from the anode cap of an old Grundig tube TV picture tube.

#### usalabs

Oct 21, 2016
10
@73's de Edd

I can't do the steps you suggested [quote = 73's de Edd] Have proper speakers… hooked up to the proper terminals on the amplifier and then use minimum volume setting,[/quote] because the volume controls either the master or channel volumes have no affect on the steadily rising crackling, and it gets to a point that it's way to loud, but by then, those power tubes have got to a point that the orange is very bright, so I immediately power down the unit.

There is not enough time to keep the unit powered on while testing the circuits.the only other way I can test the circuits is with the 4xEL34's removed, but removing the tubes would alter the outcome of the tests, and the data spec for the amp show the voltages and currents while the amp is under load and the tubes are working to spec.

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#### 73's de Edd

Aug 21, 2015
3,636
Sir usalabs . . . . .

OK . . . .then, try this, set up to INITIALLY monitor the voltage between B and C and then LOG the negative
voltage being present there.
Rapidly advance the bias pot to ascertain the max available bias and bring it back down to negative 45 if
you are tubed up with EL34's as outputs.
All of this should easily be possible before the tube filaments have even brought the tubes up to operating
temperatures.
Then . . . see if the " cherry plate syndrome " is still being existant.

If not, we more closely investigate C100 and C 104.

73's de Edd

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#### davenn

Moderator
Sep 5, 2009
14,296
73's de edd has the schematic in a lower post.

seems that has different tubes ... I dunno if your EL34's and 6L6's are the same maybe they did a change at some stage ?
it also seems to be missing a B+ supply ??

What schematic have YOU been using ?
Did you consider my comments re the output transformer ?

#### 73's de Edd

Aug 21, 2015
3,636
This amplifier design has had an evolutionary transition in the output tubes that it uses.
It started with an option of either using the metal clad 6L6 or curvy 6L6 .
That then stepped up to the use of the straight side 6L6 GB.
Then the upgrade to the 6L6 GC . . . .or its industrial . . . . .5881.
Prepare for the British invasion with the influx of the KT66.
Finally the 6CA7---EL-34 utilization.
Note that there is an S7 switch for rough transitioning between the two tube families.
Each family requires a slightly different 1st grid biasing level.
That is why I requested the other readings, nothing beats computing the actual quiescent
current of the output tubes, and setting it up.

Re: B+ power supply schematic . . . .omitted . . . .due to the sizeable area already present on the page.

73's de Edd

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#### usalabs

Oct 21, 2016
10
seems that has different tubes ... I dunno if your EL34's and 6L6's are the same maybe they did a change at some stage ?
it also seems to be missing a B+ supply ??

What schematic have YOU been using ?
Did you consider my comments re the output transformer ?

The amp uses 4xEL34's as box standard and can use the 6L6's, I believe the 6L6's are 60W a pair, whereas the EL34's are only 50W a pair.

I have the same schematic 73's de Edd posted.

Regarding the output transformer, seeing as I can not get the exact manufactures technical details for that amp, such as tube characteristics and data, and transformer specs, I can't say by testing the windings of the output transformer, that the tests correspond with the manufacture details, but a continuity test didn't show an open circuit, the test was performed with the transformer on the bench.

#### duke37

Jan 9, 2011
5,364
Take out the EL34s and measure the grid voltage. If it is not around -40 or -50V, then look at the way it is produced as has been mentioned. The voltage will not differ greatly when the valves are replaced since the current drain is low. This is of course different for the screens and anodes.
You are not looking for accuracy, something is very, very wrong.

#### 73's de Edd

Aug 21, 2015
3,636
Sir usalabs . . . .

With something that you positively should relate to:
If you don't breathe . . .you die . . .PDQ.
Since those EL34's cathodes are direct to ground . . . . thereby with zero possibility of self biasing.
If they don't have -45 volts grid biasing being provided at their 1st grids . . .they die also . . .PDQ.
(Which you already are evidencing the onset of . . . .as they presently are.)
Sooooooooo get back to them and get -45 1st grid bias on them . . . . .and then see if things don't cool down.

My questioning of the DC resistance of the output transformer was NOT being relevant to any open connection, but to be used in monitoring the actual quiescent current of the EL34 output tubes later, in their proper bias set ups adjustment.

73' de Edd

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#### grumps

Jul 11, 2015
12
wire ? a 150 watt ( or higher ) incandescent lamp - in place of the mains fuse :

so as tube heaters filter caps etc . power up ( by watching lamp filament glow )

you can limit damage / or safely short things / without burning your amp up ;

a 60 watt lamp - used in place of the mains fuse - will cut the available current

to safe level and allow troubleshooting ( ' accidental ' shorts might startle ! but

soon as they are removed the current limiting lamp should dim , some ): |

---

if bias circuit remains negative ( - 50 vdc measured at each output tube 's pin # 5 )

yet power tube plates get redder ... your amp may be oscillating , at full power ?

any inexpensive scope can show this , but human ears probably cannot hear it ;

---

latex gloves ? may add some ' shock therapy ' prevention - if you 're squeamish

about getting whacked ! by hundreds of volts .

***

#### Colin Mitchell

Aug 31, 2014
1,416
It is the valve its-self that produces the negative voltage on the grid and if the coupling capacitor is leaky, this voltage will be less negative than it should be and may even be positive.

#### grumps

Jul 11, 2015
12
It is the valve its-self that produces the negative voltage on the grid and if the coupling capacitor is leaky, this voltage will be less negative than it should be and may even be positive.
---
a cathode resistor - to ground - will develop negative bias ... at the control ( g1 ) grid ;

as cathodes are grounded ? a negative bias must ! be supplied to control ( g1 ) grids ;

( output coupling capacitors should have been replaced , at first sign of overheating ) ;

EL34s are pentodes , providing earlier linear breakup ( harmonic CRUNCH ! ) but 6L6s

are beam - power tetrodes : power ? and tone ! determine where the rabbit hole goes .

#### usalabs

Oct 21, 2016
10
Hi everyone, sorry I haven't replied for a while, and no I wasn't in the hospital with electric shock lol, even though I have had 100s of volts through me many a time in the past,,,,,at one time, I even had over a thousand volt discharge throw me on my back from the anode cap of an old tube TV, even when it was powered off, soo I'm no stranger to jolts lol, anyway, I found the problem it was the diode and capacitor that's across the 12AX7 output (via R103) to ground in parallel, CR101 and C101, the diode was short and the capacitor was leaking, I replaced both the diode and cap, and now it all works great, except there's a slight feint 60hz (USA) hum, only noticeable when the amp has no input, and the hum does not increase with volume, the hum was measured with an oscilloscope on the speaker outputs under a 4ohm load and no input and all volumes to zero.

#### 73's de Edd

Aug 21, 2015
3,636
.

Sir usalabs . . . . .

I found the problem it was the diode and capacitor that's across the 12AX7 output (via R103) to ground in parallel, CR101 and C101, the diode was short and the capacitor was leaking, I replaced both the diode and cap,

(All right . . . . some "modifier" doesn't seem to like my attention rising "RED" highlighting of quotes so " get yourself some of that" . . . . '.tater chip . . . . . . . . . local personal friend.)

Well looks like only HALF of a PROPER repair job is done,so far .
Now go to C104 and float its lead that is NOT connected to the other plate of the !2AX7.
Connect a DVM in DC 500V range to that free lead of the capacitor and the other meter lead to ground
Power up and wait for the filament warm up time and see if any DC voltage leakage reading is being present.
IF not, start ratcheting the meter on down to ever lower DC voltage scales eventually ending up in the millivolt range.
I reject any capacitor for use in this critical application, that has any more than 100-200 mv of DC leakage.
To positively confirm . . .get on the scale that has the highest . . . in range /on scale reading
Keep the probe on that cap lead connection and take a double clip test lead, placing one test clip to ground and bring the free clip up and then temporarily short the cap/meter shared connection to ground.
The existing reading will then plummet to zeee-roooo and then you watch the voltage build up and see if it is the same as before.
Use the same leakage voltage level standard / criterion as mentioned before, in selecting a new replacement cap.
Wouldn't hurt to go back and test your old and replacement cap that you had just done previously.
Final action . . . .upgrade the two 1N4004's now being used , up to 1N4007's.
Check that your supplied bias is up in the negative 43 ----45 range, or inquire for my earlier mentioned procedure where I actually take quiescent current measurement of the EL34's.
PLUS letting you evaluate the degree of matching of the pairs, as well.

Thassssit . . . . . .

73' de Edd

.

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