Low volume in preamp stage

[Realist2]

You are speaking of lack of gain of the first stage, I believe you are probably looking at input signal at the input jack rather than at the grid when you are calculating gain. I believe those grid resistors may be responsible for your signal loss.
Try this: inject your sine wave at the input jack. Measure the peak to peak volts with your scope at the following locations:
input jack
V1 pin2
V1 pin1
Alternatively, just replace the 150K resistor with a 1meg and see what effect it has.

I'm out of town at the moment, and will get back on it shortly. Thanks all.

[Realist2]

I made a punch list from my reply's, thanks

* (JM Fayhey) Bypass cathode resistors.

I had 22uf 35V caps on hand. Using three of them I bypassed R5, R30, and R13, cathode resistors.
This brought the volume up where I would expect a fifty watter to be.
It also introduced motor boating in channel 2 starting a about seven on the volume.

* (not that old, G1) Change grid resistors from 150K to 1meg.
I haven't done this yet.

* (Pedro Vecino) Value screen grid resistors 470K ?
Yes they are 470K's

* (Jazz P Bass) AC ripple on power supply nodes?
Yes I'm seeing 7V AC, at the first node right off the rectifier. And 1V AC at the second node.
The last two nodes were clean.

[Pedro Vecino]

Value screen grid resistors 470K ?
Yes they are 470K's

I also see that the power cathode resistor has a lower value than usual. As curiosity: How many volts you can read through it?

[Realist2]

Hello Pedro,
The cathode resistor (R27) of the output tubes is a 150 ohm 10 watt. Volts there are 25V DC.

BTW, I found changes in the motor boating on Ch2, when I probed it with a chop stick.
I resoldered all the joints on that board with no results.
I could increase and decrease the oscillation with lead dress.
I found a touch sensitive cap in the tone stack against the circuit board, and pryd it clear and the motor boating instantly stoped.
I redressed the leads in the other channel just for fun.
So far it is sounding really good.

[Dave H]

What would you think may be preventing the tubes full amplification in that first stage Enzo?

It will be fairly low gain because the 270k plate and unbypassed 3k9 cathode resistors give it a high output impedance which has to drive a low value (22k) slope resistor.

[Pedro Vecino]

25V : 150 Ohm = 166mA
166mA : 2 = 83mA
83mA x (390-25V) = 30 watts/tube

I've never seen a setup like this with low cathode resistor and very high screen grid resistors. Some objective should pursue..

[Dave H]

25V : 150 Ohm = 166mA
166mA : 2 = 83mA
83mA x (390-25V) = 30 watts/tube

I've never seen a setup like this with low cathode resistor and very high screen grid resistors. Some objective should pursue..

I don't believe the screen resistors are 470k. There is no way a 6L6 would pull 80mA with 470k screen resistors.

[g1]

I don't think they could be 470K either. That would be like leaving the screens disconnected.
I think you would find they measure as 470 ohm, not 470K ohm.

[Realist2]

Another problem with this Schematic that I didn't catch.

I have two of these amps and I know my 69, has 470's in it because it came with none.
Glen at Mars Amp repair in the post "Montgomery Wards squealing" back in 06, said to add the 470's, which fixed the amp.
I didn't have the schematics at the time.

[Justin Thomas]

I'm becoming curious as to who drew up that schem... it's CLEARLY an attempt to stop the cloners by intentionally filling it with "mistakes..."

Justin

[Realist2]

I'm looking at the 71 amp now, and those resistors are Yellow, Violet, Yellow.

[g1]

What do they measure with an ohm meter?
Is this the amp that has 25 volts at the cathode resistor?

[Realist2]

Yes 25volts at the cathode resistor. 510,000 ohms and 479,000 ohms.

[g1]

Maybe they are not wired up like the schematic shows?
What is the voltage at pin4 of the power tubes?

Have you tried raising the value of the grid resistor of the first triode yet?

[Realist2]

I can't power up at the moment, I have the channel 2 preamp disassembled.

The screens are tied to the #2 node, 383Volts DC at that node.