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scott17 3rd September 2012 07:50 PM

5CZ5/5BQ7A single ended amp
3 Attachment(s)
I wanted to make a stereo SE amp as small as possible. I had a few 5CZ5 and 5BQ7A that I decided to use. I used a dropping resistor for the heaters, but it would be easier to use the 6CZ5 and 6BQ7A. The 6CZ5 are $7, the 5CZ5s are $3 at The 6BQ7A are the same price, $3, as the 5BQ7A.

I don't think I could construct it to be much more compact than it is. The amp sounds very good. Less than 1mV of hum on the speaker terminals. I would guess it's about 5W per channel. I am very surprised with the response of Edcor XSE10-8-5K output xfmrs. Never used them before. Not too bad. The Antek toroid looks pretty good with the steel cover also.

I used a CL80 thermistor for the inrush current limiter because that's what I had. I think the CL120 may be more appropriate for this design.

This amp can be built for about $140, buying all the parts new, not including the chassis. I used some funky trim material I found at HDepot and a piece of scrap aluminum.

soulmerchant 3rd September 2012 08:49 PM

Nice little amp! you can by-pass the UF4007's with inexpensive low value film capacitors (use one for each diode) to help smooth the edges off the ripple. Better yet source a choke to get get the last of that hum.

scott17 3rd September 2012 10:22 PM

Thanks. Ha! I don't think I could fit a choke in that box!

My loudspeakers are 97dB and I can't hear a thing. The amp is dead silent.

I am interested in the bypass caps for the bridge though. What would you suggest? 0.01uF?

silcostoni 4th September 2012 12:14 AM

Look great ......i hope ,,i can get This Tube in Indonesia :)

scott17 4th September 2012 01:51 AM

FYI. The red LEDs that I used are just standard devices I had around and matched to achieve a 3.5V drop.

wa2ise 7th September 2012 01:25 AM

With the 5V heater tubes, you could use an old PC computer power supply to heat them. Of course you still need to build the B+ supply. :)

scott17 7th September 2012 01:38 AM


Originally Posted by wa2ise (
With the 5V heater tubes, you could use an old PC computer power supply to heat them. Of course you still need to build the B+ supply. :)

That is true, however look how easy it is the way I've done it to keep it compact.

Besides that the heater voltage for 5CZ5 is 4.7V. The heater voltage for 5BQ7A is 5.6V.

The 5CZ5/5BQ7A combination sounds fantastic. I say it rivals an EL84/anything driver.

Inexpensive too.

scott17 20th October 2012 01:24 AM

The more I listen to it, this is a fantastic sounding amp, and very inexpensive. If anyone would like the Mouser parts list please PM.

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