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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
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Old 14th October 2019, 02:47 AM   #101
Kofi Annan is offline Kofi Annan  United States
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I've always connected the grounds from the power stage, preamp stage and input grounds to the chassis near the first PSU cap, then connected that chassis-grounded star to the chassis safety ground next to the IEC inlet along with the earth connection. Two chassis connections.

I've never had a problem with hum, but now that I have a preamp in the mix things may change.

Are there safety concerns if I don't attach the preamp / power / audio grounds to the chassis?

Kofi

Last edited by Kofi Annan; 14th October 2019 at 03:17 AM. Reason: Removed run-on bananasentence.
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Old 14th October 2019, 02:32 PM   #102
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To clarify, I ground the loop breaker to the chassis using a different connection point than the AC safety ground. The safety ground has it's own connection.

It's good that you have not experienced any previous hum problems, however there should be only one connection point from signal ground to chassis ground. Whether you use a loop breaker or not, it is possible to develop a ground loop within the chassis itself which may show up as hum.

Yes, there are safety concerns if signal and chassis ground are not connected. Some type of catastrophic component failure could put HV on the signal ground. Also, grounding the chassis minimizes noise and hum.

I hope this is helpful.
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Old 14th October 2019, 06:15 PM   #103
Kofi Annan is offline Kofi Annan  United States
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Scott,

Yes! Thanks for posting. For now, I'll stick with my method and ensure the master star is attached to the chassis.

Also, after further examination it appears that I'm running the EL84s past max dissipation and voltage (not that much, but still...). I should probably drop the B+ to 300V with a resistor.

I know that other designs run EL84s hotter, but I'd be concerned about long-term reliability and general failure.

Am I being too concerned here?

Kofi
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Old 14th October 2019, 09:40 PM   #104
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I've seen other designs as well that run the plate voltage higher in pentode with the screen at a lower voltage. I don't have a lot of experience, any actually, with the EL84 in parallel push-pull pentode, but I remember reading on this forum somewhere that original production EL84/6BQ5 were better at handling the higher voltage with a well regulated screen.
I'm guessing you will be using new production tubes. If it was me, I'd stay within the published specs if you are looking for long-term reliability.

What is the final configuration for your new amp? Pentode wired cathode bias? Individual cathode resistors?
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Old 14th October 2019, 10:59 PM   #105
Kofi Annan is offline Kofi Annan  United States
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Here's the final design. Note that the voltage is 325 in the schematic, not 300. Reducing the voltage will negligibly increase distortion and reduce output current, but would pull the EL84s back in line with max operating voltage and power dissipation.

For the record, I'll be using some NOS Russian mil spec equivalents (matched octal) that should be rugged enough for the task, however it's possible that the client changes does some tube rolling in the future with less durable models and winds up red plating.

I do currently run a Baby Huey EL84PP with about 320 on the plates, but since this will wind up with a client, I think it would be wise to ratchet it back a bit.

Kofi
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Old 15th October 2019, 02:59 PM   #106
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That's going to be a nice amp. That's a great selection for the OT. If I was going to keep it I'd leave it like it is power supply wise. I mean it's right on the edge after taking into account the cathode voltage. But as you say, you can't tell what the client may do. I'd definitely not make any changes to the power supply until you have it powered up and see where you actually are.

Last edited by scott17; 15th October 2019 at 03:01 PM. Reason: additional info
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Old 15th October 2019, 03:31 PM   #107
Kofi Annan is offline Kofi Annan  United States
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Thanks! You should know that the design was a team effort (kevinkr, Lingwendil, rongon and others with rongon working up the final design in LTSpice).

I'm hoping to complete the wiring by tomorrow sometime. I'll post after the initial power-up with a list of all the components I blew up and a request for a local therapist's contact info.

Kofi
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Old 17th October 2019, 10:35 PM   #108
Kofi Annan is offline Kofi Annan  United States
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OK, so I powered up tonight with a variac and at about 60VDC out the PSU smoked two 20W resistors in series and popped the fuse. I disconnected the PSU from the circuit and ran a 1.2K 20W resistor across it, which it promptly smoked at around 70 VDC.

If there was a short in the supply, it should have popped the fuse but not fried the resistor, so I'm wondering if there's something up with the transformer. I wired the red leads together and the black leads together on the primary side (wired in parallel) for 115V and grounded the purple wire, which I believe is for the screen.

I'm about out of brain cells for the evening, but any advice would be much appreciated.

Thanks,
Kofi
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Old 17th October 2019, 11:40 PM   #109
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Are you sure those 20W resistors are actually 1.2K? At 70VDC the resistor would be dissipating just 4W, consequently it seems unlikely that that is the resistor value.

But I wonder why you are doing this at all?

I would normally apply power to the entire amplifier and measure all voltages and currents starting at a small fraction of the rated line voltage. You can also use a ballast lamp in series with the primary to minimize mischief in the event of a mistake.
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Old 17th October 2019, 11:44 PM   #110
Kofi Annan is offline Kofi Annan  United States
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Bah. The PSU is rated for 660ma when in parallel, hence the smoked twenty-watter. Itís possible that I have a short somewhere that caused the B+ dropping resistors to smoke. Also wondering if I brought up the variac too slow.

Kofi

Last edited by Kofi Annan; 17th October 2019 at 11:47 PM.
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