Danelectro DM25

[J Luth]

Recent purchase, his complaint appears to have low output, not much gain and a bit of hum.

Looks like it spent a lot of time in a dirty damp basement. Faceplate has a lot of surface rust, reverb supports rusty and the inside where the pots are looks like the inside of a desktop computer that hasn't been cleaned in 10 years.

Took a look and checked a few things after cleaning it. Almost every resistor was out buy 30 to 40% and a bad cap on the reverb tube.

Work done.
New filter caps, all bad resistors and a couple that were wrong according to the schematic and caps replaced, for lack of a better name the death cap removed, 3 wire cord added and all new tubes.

Amp sounds good now nice and quite, tem works well even reverb is working but it may still have output issue. With the volume all the way up and playing through it, it is not all that loud. When checking the output of the amp it comes out to 12.5 watts. Would that be correct or should it be more? Haven't been able to find out what the output should be.

[The Dude]

Regardless of output power, I would think you'd get some breakup way before you hit max on the volume knob. Here's a schematic with voltages. See if anything measures vastly different from specified voltages.

https://el34world.com/charts/Schemat.../Dano_dm25.pdf

[g1]

I think the 6X4 rectifier will be fairly limiting as far as output power.

[The Dude]

Oh, I agree. It still seems the amp shouldn't be totally clean at max volume, which is the impression I got from the 1st post. It wasn't implicitly stated that way though, so maybe I'm reading into it.

[g1]

Yep. I was thinking 'also' but I should have stated it.
Full tilt should make it crunch and 'sound' loud regardless of power output.

[xtian]

Did you try playing thru an external speaker? I'll bet the one in the combo is not very efficient.

OTOH, amp should distort when Volume is maxed. Here's another thread on the DM25: https://ampgarage.com/forum/viewtopic.php?t=11649

[J Luth]

Oops should have been more specific.

I have the schematic, I'll recheck voltages.

The Dude
Yes it is clean with the volume all the way up, no breakup.

g1
No crunch sounds wimpy

xtian
yes I did try an external speaker with the same results.

[Chuck H]

Oops should have been more specific.

I have the schematic, I'll recheck voltages.

The Dude
Yes it is clean with the volume all the way up, no breakup.

g1
No crunch sounds wimpy

xtian
yes I did try an external speaker with the same results.

What was the input signal? And if it was a guitar (which anyone would expect) was the volume knob on the instrument turned all the way up?

[J Luth]

Chuck H

First checked on the scope with a 1k 100mV signal, scope set to 2v PD wave form peek to peek a bit over 5 div's. The peeks did start to flatten out just before it reached max volume and showed no notch distortion.

The sound test was done with a guitar and yes the volume was turned all the way up.

I have been guilty of not having the volume turned up before.

[J Luth]

Did some voltage checks and most are within + - 10 to 20 volts except the first 2, 12AX7's. Plate voltage is high on all plates.

Voltage shown on this schematic.

[Helmholtz]

The very low 7189 plate voltages are not plausible. It would mean that the OT drops around 50V, which in turn would require an idle current of 150mA or more (depending on OT DCR).The tubes wouldn't survive such plate dissipation of over 40W each.

Are you sure you measured from tube plates to ground? Please also verify 341V at the OT center tap.

[J Luth]

Helmholtz

My mistake on the 7189 plates I'll admit to it, I read the wrong pin DOH. It was late.

Using a Fluke 117 meter

After full warm up

7189 plates 334 and 335

Output trans center tap 340

[J Luth]

Rechecked everything today and with the exception of the voltages listed above all others are the same as shown on the attachment.

I did find two caps on both tone stacks that had there leads shorted against the pots. Moved them and the amp sound better and now has some breakup on both channels when the volume is turned up.

Should I be concerned about the plate voltages on the 12AX7's?

[xtian]

Sounds like you solved your problem!

Did you replace the 25uF cathode bypass caps?

[Helmholtz]

Should I be concerned about the plate voltages on the 12AX7's?

Plate voltages being up to 20% high are not suspect. Higher than that may be caused by drifted cathode or plate resistors - or just by a weak (decrepit) low gm tube.