harmony 415c trem help

[riz]

Working on this for a friend. D.C. voltages on the 6au6 are as follows:

pin 1: -0.47v
pin 2: 0v
pin 3: 0v
pin 4: 0v
pin 5: 69.8v
pin 6: 44.7v
pin 7: 0v

There is A.C. heater voltage present and the tube lights up. Tried two other replacement tubes (from Antique...) and they yield similar measurements.

Link to schematic: Prowess Amplifiers - Misc - Schematics - Harmony 415

Amp has had electrolytics replaced, and voltages (other than this circuit!) are reasonable, if not surprisingly close to the schematic! It sounds great, but the trem won't work. (didn't work before the re-cap either...). I have tried it with a pedal, and have swapped out all the caps in that part of the circuit, and changed several bad resistors, all to no avail. I'm now in over my head, so any help looking at this problem is greatly appreciated!!

[Jazz P Bass]

First thing that jumps out at me is the plate voltage. Pin #5.
The schematic shows 160Vdc.
Your reading indicates 69.8Vdc.
You may or may not have any heater voltage.
Can you retest Pin #3 to #4 with your meter set to read Vac.
I would go back & recheck the work that was done.
Verify all of the component values.
And most importantly, verify that the layout agrees with the schematic.
Check everything, point to point.

[riz]

Yes, there is heater voltage. Today it's a little high at 6.5v a.c. Over the weekend last time i looked it was 6.3. The plate voltage seems very wrong to me, too. Like the tube is pulling too much current...? I will recheck the layout again for mistakes. I've been over it several times, it's a bit of a valco-looking rat's nest. I found several problems with way-out-of-spec resistors, each time thinking"that's it!", but no luck yet! The 150k and 100k dropping resistors are good. I will revise the list of what I've changed, with more voltage measurements. Would it help ya'll to have voltages with the tube pulled?

[melvin]

Make sure that socket is clean and tensioned. Make sure the connections to the grid from the PL resistor are viable. That would surely cause the tube to draw too much current and fail to oscillate.

[riz]

I re-tensioned the socket already. Which is the PL resistor? (what does PL mean?) The 390k?

[melvin]

Plate Load. The resistor connected to the plate (pin 5). Dirty socket or bad solder joint could cause the bad connection.

[RWood]

Prior to your plate load resistor R29 and R28 before it are you getting the called-for 280VDC? Since your screen voltage is only 10V low I would guess that you are, or something close. If that is the case, R29 and R28 are possible culprits, along with anything way out of spec in the oscillator circuit, R32-35, C14-16. You mentioned replacing all of that; if so something could be grounded out or you could (still) have a bad component.

Your grid voltage, btw, is also only about a third of what it should be at -.47 versus -1.4. So I would focus on what is connected to pins 5 (plate) and 1 (grid).
Note that the trem tube voltages on the schematic were measured at maximum speed.

Your footswitch jack is open, right?

[riz]

Yes, I'm getting the 280 volts. I haven't had time to mess with this the last couple days. One thing I haven't done is spray out the socket, but I did re-tension it. I will do that, and follow up on the other suggestions before I report back. Thanks!

[riz]

WORKS!

Thanks for the help. Turned out to be one of the .022 caps wired wrong. I can only take the blame for that. I imagine it works as it was designed, it's quite nice, but how do you increase the intensity?

[melvin]

I'm glad it's working. To increase the intensity, I would first check that the 220k resistors and C18 are good, that the foot-switch jack and intensity pot are working properly, and that R28 and R29 are new and correct. These resistors can be adjusted to give more intensity to the tremolo, but I would do so conservatively as you are messing with a bias voltage on the grids of the output tubes. However, there is a degree of safety built in considering the amp is cathode biased, not to mention that the bias voltage is alternating.

So, if you decide to try it, just make R28 bigger and R29 smaller while keeping their total resistance the same.

[riz]

Thanks melvin! I suppose the obvious thing to do would be flip R28 and R29... I will try it. It's fine as is, just want to take the opportunity to learn something while I'm in there. I added 100 ohms in series with the 130-ohm cathode resistor, it was getting a nasty distortion and the output tubes were pulling 15 watts each at idle. Around 11 watts now and happier. Now I'm curious about screen grid resistors... maybe that's another thread!

[melvin]

Did you mean parallel to the cathode resistor? Speaking of the screens, I wonder what that extra wire is doing going from the screen pins to one end of the output transformer primary in the schematic. I believe that's a mistake. Also, the screens is the last thing I would mess with. There are plenty of other more interesting things to muck around with...

[riz]

No, in series, jumpered in as a test. Increased the 130R to 230. 200R wasn't quite enough. Yep, this is a really cool amp, wish it were mine!