Harp amp design advice

[SeddoniaPhil]

Hi there,
I'm hoping for some reviews of my schematic - any advice appreciated
I'm building a harp amp based on a 5c3 and 5d3 Deluxe - keen on trying out the grid bias from the 5c3 (for crystal mics on channel 1) and paraphase inverter from the 5d3. Channel 2 is more based on a 5f2 input (but likely will use a 5 MEG input resistor). Using .1uF caps throughout, both resistors for v1 will be 150K.
Cheers
Phil

IMG_8933a.png

[Stevem]

One of the main things I have heard about Harp amps and having them not be as prone to feedback is to wire the speaker out of phase.

All the great tone in the world does not matter if the player can’t be heard!

[pdf64]

Harp players definitely can’t be heard after being electrocuted. Wire the mains properly, fuse in the live feed then a dual pole switch, and a solid safety earth connection to the amp’s chassis.
They were only designed like that originally due to the 2 pin unpolarised mains connectors in use then.

I suggest to select a mains transformer with a ‘lower than typical for a 5E3’ HT winding voltage (at HT V DC at the reservoir cap - OT CT node of around 360V makes keeping dissipation reasonable much easier). And the capability on the heater winding to use 6L6 output valves.

A balanced heater supply would be very beneficial in reducing heater hum / buzz. Referencing the heater winding to the output valves’ cathode is a good method to further reduce / eliminate heater buzz (makes the amp less fussy about selecting valves with very low heater to cathode leakage).
As per this Gibson GA20 https://el34world.com/charts/Schematics ... preamp.pdf

[SeddoniaPhil]

Hi all. An update and request. I've finished the build and started up (layout and initial voltages attached). I'm using a Hammond 290BEX power transformer and 1750H output transformer. I've found I'm getting no sound and 785v on pin3 of 6v6 plates (no red plating though). I've checked all solder joints are good. Robinette's troubleshooter indicates high voltage can be from either no heater voltage or incomplete grounding of the cathode. I've checked both and they appear to be ok. Any suggestions for where I should go next error checking wise? Thanks heaps, Phil https://drive.google.com/file/d/1E5EAXw ... drive_link

[martin manning]

With the 5Y3 removed, measure AC voltages from pins 4 and 6 to ground, and from pin 2 to pin 8.
Do I understand correctly that you measured 364V DC on pin 8 with the 5Y3 installed, and 785V DC on the power tube plates?
What was the DC voltage on the cathodes (pin 8's)?

[SeddoniaPhil]

Thanks for your reply.

No tubes installed.
Rectifier
pin4-chassis =363
Pin6-chassis=363
Pin2-8=5.2

I didn’t measure cathode voltage as I saw the excessively high voltages, so paused further investigation to find out how to address. I can check if it helps?

[martin manning]

Voltages on the rectifier pins look good. What resistance do you see from power tube pin 8's to ground? Should be the 250 ohm cathode resistor. I don' see how you can have 365 and 300 on the plate and screen filter positive leads and then be hundreds of volts higher on pins 3 and 4.

[SeddoniaPhil]

Thanks Martin. Yeah, 252.1 Ohms from 6v6 (v3) to chassis and 254.0 from 6v6 (v4) to chassis.

I’m at a loss for why the difference too. Appreciate your thoughts. Cheers

[Stevem]

Can you post up a couple of clear gut shot Photos of the ps and the output section .

[SeddoniaPhil]

No worries.
Pwr section https://drive.google.com/file/d/1VSYMfC ... p=drivesdk
Pre amp https://drive.google.com/file/d/1qAYs2R ... p=drivesdk
Inputs and controls
https://drive.google.com/file/d/1M0vOxl ... p=drivesdk

[sluckey]

Red/Yellow, red, and black are unusual colors for primary leads on an OT. Verify they are connected correctly. Easy to do in circuit. Turn power off. Measure resistance between rectifier pin 8 and 6V6 pin 3 for each tube. Resistance readings should both be about the same. What have you?

[SeddoniaPhil]

Yeah, it's a second hand OT. Previous owner had trimmed them short on the exit and then added extra length so they could enter the chassis. Red is correct, the black wire is an extension of blue and red/yellow is an extension of brown.
Red to blue (black) = 204
Red to brown (red/yellow) = 205
Blue to brown = 408

[martin manning]

How are the batteries in your meter? Sometimes low battery state can cause bizarre readings.

[Lynxtrap]

One of the main things I have heard about Harp amps and having them not be as prone to feedback is to wire the speaker out of phase.

Out of phase with the first gain stage?

[Stevem]

At the speaker.
Amp chassis ground to + on speaker(s)