• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tried failed, looking for suggestions

A few weeks ago I sought and received some help about rebuilding a Knight amp I have, especially from Eli Duttman.

Well I got in over my head. After getting started I find a couple of wires that have broken off and I can’t tell where they go, some burnt components, the schematic I have doesn’t appear to follow completely, there’s 50 years of dirt and grim to try work through. And the list goes on.

I haven’t written it off completely yet, but I’m getting there.

Anyhow I have build a few amps with much better success and I’m looking for suggestions that I could possibly reuse some of what I planning to use on this project for this project.

PT - Edcor XPWR178-120
660v 200ma ct
6.3v 6a ct
5v 3a ct

OPTs 8500 0hm ct (these are from the Knight amp)

Tubes
EL84
12AX7
5R4 rectifier

Triad C-14X choke

Thanks
 
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Hearinspace

Member
Paid Member
2008-06-03 5:18 am
If it were me, I'd probably think about whether I like the chassis and wound components enough to reuse them in a new design. These days I'm playing a lot with differential circuits and liking very much what I hear. So I might think about gutting & refinishing the chassis and using the parts to make a simpler differential power amp or possibly integrated one.

I have no idea how good the Knight output transformers are but one point has me curious. In the schematic, the output transformer is shown with what appears to be an unbalanced primary winding - 220R on one end and 200R on the other. In the upper channel the drawn diagram itself shows the offset but it's not in the lower one. I've never noticed this before, does anyone know what it's about?
 
Knightkit was always a kind of low end kit, IMHO. If you want to stay with hifi, Classicvalve.ca sells driver boards compatible with but improved from dynaco amps. With a 5AR4 you don't have to worry about the rectifier restricting your power. I'd try one of the ST70 driver boards like the three 12AX7 one. I believe kevinkr designed the KTA one. If you buy from Canada, get shipment by mail; UPS and FedEx charge for paperwork and loans through customs.
You've got enough voltage for a low powered EL34 amp, although I don't know how much B+ amps you've got. It won't be ultra-linear design with knight output transformers, but you can buy the US made dyna copies OT's from triodeelectronics.com any time you want to upgrade. Same source has more powerful B+ & heater transformers.
I've salvaged a 12" deep file cabinet perfect for a project of this size, but you may want to buy something professional looking to show off with your digital camera.
There are also a lot of guitar amp designs out there if you play electric ax. triodeelectronics.com and tubesandmore.com have some.
 
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JMO, it's "El Cheapo" time. :D

The "ideal" B+ rail voltage is 355. You can get there by using a small 1st cap. in a CLC filter. Start with 1.5 μF. and work your way up, until the loaded down supply reads 355 V. Here, the large forward drop in a 5R4 is a distinct asset.

Contact Jim McShane for the RC bias network values appropriate to EL84 "finals".

The O/P "iron" lacks UL taps. Therefore, triode and full pentode mode "finals" are your options. If you go full pentode, regulate g2 B+. Increasing the % of the O/P voltage fed back, to improve pentode mode damping factor, makes sense.
 

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Thanks for the replies.

The ST70 looks good but I built a ST35 clone a while ago and would rather do something else than another Dynaco at this time.

The Baby Huey does look interesting and is worth considering.

But I think I'm leaning towards the El Cheapo.

Hello Eli,
As you might of expected, I have a few questions.
- what changes are needed to go to triode?
- a few old post mention a huge El Cheapo build thread. The links to it are all dead. Do you know if it still exists? Can you point me to it?
Thanks
Richard
 
As you might of expected, I have a few questions.
- what changes are needed to go to triode?
- a few old post mention a huge El Cheapo build thread. The links to it are all dead. Do you know if it still exists? Can you point me to it?


The graphic I uploaded shows triode wired "finals". ;) If you wire the sockets for the 6П15П-EB (6p15p-ev), you can use both that type and 6BQ5s. Retain the 1 KOhm value for tying g2 to plate. While 6BQ5 screen grids are tough, g2 in the 6П15П, like that in the 'AQ5, is fragile.

Sadly, the Audiophile Talk site, where the saga of "El Cheapo" resided, was "hacked" and destroyed. :bawling: :mad: Fortunately, someone was able to save most of the thread. Only the most recent posts were lost. PHEW!! Scan the archives here and over at AA to get a URL.