The Red Light District - another PP EL84 amp

[woodturner-fran]

Well, I hooked up the scope. Here is where your sig line is especially true SY!

As far as I can see there is a slight sine wave, but thats with the volt div at 20mV. It seems very "small" if you know what I mean. I don't see any major waves etc. If someone can tell me what I should have the volts and time divs set at and what to watch out for, I can do that. If I measure AC at the output Tx with my DMM I get <3mV, slowly going down.

Valves are indeed russian 6n14n and there is a bit of blue glow inside the black plates. They appear brand new and have a code on them 0480 (april 1980?)

Both channels settle to 65-70mV after being on for a few minutes. Screen voltage stays at 293V.

Going to listen now - reports later!


Fran

[SY]

OK, what you're seeing is just the buzz. I suspect your issue with over-current is gas in the Russian tubes.

Pour a beer and go have a listen.

[woodturner-fran]

OK,

listening:

Sound is good and all that - everything is as it should be, soundstage, bass mids and treble. All good and transients OK etc. Does sound different to my other amp, but then thats to be expected.

I do have some buzz - and its intermittent. Comes and goes, but I think its from the 24V aikido pre. When I mute the inputs, I get silence.

With the other amp hooked in, I had hum, stronger in the right channel. Now I have buzz, again, stronger in the right channel, and as I said a bit intermittent. Its independent of volume setting. Not terribly loud, but you would notice it in quiet passages.

Probably what I need to do is hook up a source with a built in volume control and use it for a while and see how the amp goes. If its OK (and it was in the workshop) then its the pre.

So more testing needed, but sonically the amp seems to perform well.


Fran

[woodturner-fran]

Just to say the amp seems pretty stable. The buzz is definitely coming from the preamp, and is independent of volume. When I connected my squeezebox up directly to the RLD it is dead silent. It seems to be settling in nicely, been on today for about 4 hours so far and no hisses pops bangs or wallops.


So I seem to remember a link around here for diagnosing buzzes and hums (and this is raspy buzz, not hum). I searched but can't find. I think it was along the lines of buzzes come from one place and hum from somewhere else, but dang if I can't remember or find the link now. Anyone know where it is?


Fran

[feppe]

ello,

I am building this amp and recently i saw the the electronic crossover at your website. Is there a convenient way that i can inplement those two in the same box. I mean, the ecc88 buffers are at ca 100 plate voltage which means that i need a third hv-supply. If possible, how would you implement adecuate buffers using for example the 400V supply for the ecc81 in the rld?

In my hand i have two altec 15335A bridging transformers (1:1, 15kohm/15kohm). In the datasheet, these units response are flat to about 40khz but the phase response are beginning to drift at 10khz and are -50degrees at 40 khz. Would these units compromise performace as input transformers?

Also i would like to say thanks for sharing your expertise and it has been a pleasure reading the article!

Axel

[SY]

No reason why not. Just be careful to keep it away from the transformers. As a practical matter, I'd probably substitute an ECC81 for the ECC88 and increase the B+ to 250V, just to reduce dissipation in the regulator- after all, you've got about a 40mA draw, so there's 4W per 100V to be dropped.

I put mine into the Heretical box, using the original Heretical board as the second cathode follower and a modified Heretical board as the first cathode follower. At some point soon, I'll finish the next part of the article and show some photos.

I'm not a believer is excessive bandwidth, so the 40kHZ pole wouldn't scare me too much.

[feppe]

Exellent, i'll confidently include another set of sockets on the chassis.

[SY]

I changed my mind. Thinking it through, 250V at 10mA for the ECC81 takes you to 2.5W per section, a bit higher than that tube should be run. It's better to put in an extra transformer for the crossover bit or uprate the PS already there to handle the extra 40mA for the crossover and live with the 4 or 5 watts that the regulator will need to dissipate.

Don't forget the +/-12V that the opamps will require.

[feppe]

Hi again,

I was rereading the heretical article. You write that 6sn7 is an equally good choice as ecc88 but requires higher plate voltage. Therefore I speculate that 6sn7 could be a a good choice for me?

Anyhow, the ps part will be extensive with the 12V supplys and possibly a third hv supply. I probably need to make a pcb for that.

[SY]

I don't think I said that. The 6SN7 has a considerably lower transconductance, hence is not as good. It's a superb voltage amplifier, but a mediocre cathode follower.