RH84 motorboating, stereo vs mono

[fyzygy]

When connecting up B+ to the 12AT7 preamp tube in my RH84 SE build, I am experiencing trouble. Namely, motorboating and squealing.

According to the schematic at the RH84 web page, B+ goes to the preamp tube via a 10K and 22K resistor in series, the junction of the two resistors goes to ground via a 10uf capacitor.

Should I use two such resistor/capacitor networks to feed the 12AT7 anodes separately? If I do that, the motorboating starts ... in stereo. I've tried a few different caps and the best I could come up with was a signal that distorted badly on input peaks.

But if I connect 12AT7 anodes to each other, and feed them both from the one cap/resistor network as described above, I get excellent sound ... only in mono! No motorboating or squeal.

Please help! I'm going insane trying to figure this thing out. (This is my first stereo tube amp build, in case there was any doubt.)

[fyzygy]

Is this right:

The 10k resistor drops voltage.
The 10uf capacitor filters residual AC to the preamp stage.
The 22k is the plate load resistor.

This works fine for mono (B+ to both anodes). Actually, sounds fantastic. However, B+ to each plate of the dual triode gives me distortion on peaks, sharp squalls, and motorboating as amp is switched off & fades to silence. And less output power.

Tried different 12at7 tubes, not much difference.

Tried different capacitors, ditto.

My amp is running around 250 volts at this point, somewhat less than the RH84 spec. But I understand this shouldn't be a problem?

Have I just built an overkill parallel single-ended amplifier?

[costis_n]

from the power supply you have TWO 10k resistors and TWO 10uF capacitors, one per channel, right?

[fyzygy]

i did have TWO capacitors and TWO resistors (10k) and another TWO resistors (22k) coming from the power supply. but it seems that any permutation of that gives me the motorboat type nuisance.

[Soonerorlater]

Should I use two such resistor/capacitor networks to feed the 12AT7 anodes separately?

This is what Mr Kittic's circuit calls for.

But if I connect 12AT7 anodes to each other, and feed them both from the one cap/resistor network as described above, I get excellent sound ... only in mono! No motorboating or squeal.

Perhaps it would be better not to experiment with bridging stereo channels into mono until you've got at least one channel working. The only contacts between each channel should be at the junction labelled " B+ " (where each channel jointly gets powered from the power supply) and the shared earth. Working on one channel at a time might be a simpler approach. The 220 ohm resistors should be soldered close to the socket pins with the shortest of lead, helping to prevent oscillation.

Let us know how closely you've stuck to the power supply circuit. The B+ current needs IMHO to be at least 100mA dc. What's your transformer cabable of ?

What's your source?, CD, mp3 player , preamp?

Brgds Bill

p.s. If this amp had worked properly first time you would have missed out on all this fun.

....

[kevinkr]

Pictures showing how this thing is put together would help, sounds like a possible combination of grounding issues and layout problems.

Have you adhered exactly to the original design or made some changes along the way?

Also what are you using for a chassis?

[Soonerorlater]

RH 84 - Tube Audio ...... RH DESIGN

For reference

[fyzygy]

My circuit is same as RH84 web page, except I have 240R instead of 250R cathode resistors on 12AT7. My power supply uses a 560-ohm 5W resistor instead of a choke - so I'm getting around 260v rather than 300v. Filter cap values are equivalent to the original circuit. My grid resistors were soldered directly to tube sockets, nice and close.

Enclosure is an aluminium box, with star grounding scheme.

I have been using an iPod as my test source signal. Is that a bad idea? That's what the amp is intended to amplify.

Is it preferable to have separate B+ wires running from power supply to the various destinations? Or can I simply run a wire to first OPT, and from that point to second OPT, etc.?

I will double check all wiring, see if I can get at least one channel to work as it should. Thanks for the input. Any further ideas would be welcome!

[kevinkr]

Take some pictures of the internal layout if you can.. Particularly interested in the grounding scheme.

[Soonerorlater]

Is it preferable to have separate B+ wires running from power supply to the various destinations? Or can I simply run a wire to first OPT, and from that point to second OPT, etc.?

There can be only one destination point per channel for the B+ supply and that's the common point in the amp circuit labelled " B+ ". To keep things simple I would only supply one channel. When you have resolved your problem then you can think about the most practical way to power both channels.

In my amp I ran a cable from the power supply to one channels B+ supply point, and then a short link to the second channels B+ supply point. This was convenient with no apparent noise issues.

HTH Bill