5F6A type amp drawing too much current

[six el six]

This is a "boteek" 5f6a with el34s, adjustable bias, adjustable negative feedback.

It's drawing too much current at 3/4 or so up when you hit a wailing note in particular.

Any recommendations? 2amp fuse. Meter just goes over two amps when I dig in.

Voltages attached and pic.

Thanks,
six

[PRR]

Check load impedance.

[six el six]

Check load impedance.

8 ohms

[DummyLoad]

if you have a bassman "replacement" OT, and it's a single secondary, it's likely wound for a 2ohm speaker load. it that's the case, then you have 4 times the reflected impedance (16Kohm) with an 8ohm load connected to the secondary. could that be the problem?

--DL

[six el six]

This is a Louis Electric Amplifier KR12.

I'm inclined to believe it's supposed to be seeing 8 ohms.

It has grid stoppers of 6k.

[tubeswell]

It doesn't matter what you believe about the amp - the only way to know for sure about the load resistance, is to measure the impedance ratio of the OT in order to work out speaker load that you should be running in order to reflect the optimal load resistance to the output tubes.

[six el six]

It doesn't matter what you believe about the amp - the only way to know for sure about the load resistance, is to measure the impedance ratio of the OT in order to work out speaker load that you should be running in order to reflect the optimal load resistance to the output tubes.

Good point. Thanks tubeswell.

Applying 1 volt to the primary at 1k I get .047A on the secondary. This gives me a voltage turns ratio (VTR) of 21.766.

The VTR squared is 452.694. If I understand correctly this is my impedance ratio (IR).

IR x 8 = 3621.55

IR x 16 = 7243.1

IR x 4 = 1810.78

IR x 2 = 905.388

Now, is ELR (effective load resistance) the same as Ra-a as I see it in tube data sheets? In the Phillips data sheet I see 3.4k and in the Mullard sheet I see 7k.

I need help interpreting this data.

[sluckey]

It's drawing too much current at 3/4 or so up when you hit a wailing note in particular.

What does that mean? How much current and how are you measuring?

If you mean the output tubes then have you tried adjusting the bias? Or changing the tubes?

[six el six]

.

What does that mean? How much current and how are you measuring?

If you mean the output tubes then have you tried adjusting the bias? Or changing the tubes?

I'm measuring current from the wall and current through the power tube cathodes.

With the bias pot adjusted for approx. 30mA per tube (reasonably matched) the current draw from the wall is .74 amps.

It blows fuses when it's turned up three fourths or more and I hit the guitar hard. I can watch my current meter in line with my wall ac swing just up to 2 amps and of course the fuse blows then.

I've tried different power tubes = same thing. Turning the bias pot too far down just results in crappy tone and it still blows fuses turned up.

It's almost as if it needs a half amp larger fuse? Is that possible? I've been getting as much as 125vac out of the wall lately.

[tubeswell]

Applying 1 volt to the primary at 1k I get .047A on the secondary.

You need to measure the VAC on the primary and the secondary to get the VAC ratio

[six el six]

You need to measure the VAC on the primary and the secondary to get the VAC ratio

Ok.

I just followed your instructions from another site and I got a VAC of 20.8:1 which gives 432.64:1.

If I understand correctly hooking up an 8ohm speaker gives me 3,400 pr load resistance.

Vp-p for el34 in class ab is 3,400 correct?

Yes?

[six el six]

A new rectifier tube seems to be correcting the problem.

Now when the amp is cranked it swings just over 2 amps but the fuse doesn't blow. If I understand correctly this is because the fuse is "slow blow" and they don't blow as fast.

Has anyone else ever had this problem w/rectifier tubes? It's a JJ GZ34.

[tubeswell]

You need to measure the VAC on the primary and the secondary to get the VAC ratio

Ok.

I just followed your instructions from another site and I got a VAC of 20.8:1 which gives 432.64:1.

If I understand correctly hooking up an 8ohm speaker gives me 3,400 pr load resistance.

Vp-p for el34 in class ab is 3,400 correct?

Yes?

Correct