The good, the bad, and the ugly OTL amplifiers

[Original Burnedfingers]

Ok, I'm looking to buy a OTL amplifier at a good price. I need information in order to formulate my decision of to buy or not to buy. I need your opinions, facts, judgements and so forth. Please convince me one way or the other here.

I have been told they are dirty little buggers that cannot be trusted. Some tell me they go thru tubes like they are going out of style and yet others tell me they can be trusted to deliver good and trouble free service. Some tell me they have no bass response and yet other people rave about the low end.

Please help seperate fact from fiction here.

[SY]

Number one: what speakers are you using? OTLs tend to be VERY finicky about their loads. If you answer "Quad ESL," then you're a candidate.

They are horribly inefficient, tend to be unreliable, and eat output tubes. I've owned some original Futtermans (Futtermen?) and a home-made monster using 16 6528. Interesting experiences, but I'll never do that again.

[Tweeker]

Some of them are pushing their tubes way beyond their long term limits at full power into some loads.

6C33Cs have the potential for thermal runaway if care is not taken.

[Original Burnedfingers]

SY,

Thank you for your reply. I have an upgraded version of the Klipsch Cornwall for speakers. Was that 16) 6528's per channel?

I'm looking at (10) 6as7GA's per channel on the amp I am looking at. Has shutdown circuitry as well for faults.

[SY]

Yes, 16 per channel. The heat load just from the output stage heaters was nearly a kilowatt. It was a very inefficient, inelegant design, but a friend had been given 64 6528 out of a NASA shake table and wanted to do something with them...

What does the impedance curve of those Cornwalls look like?

[tubetvr]

6C33Cs have the potential for thermal runaway if care is not taken

I have never heard or experienced that, can you tell me more? I have used 6C33C in OTL amps for many years but never had a serious problem. As a reliability test I was running one amp at full power for 24 hours without anything bad happening except that everything got very warm, I seriously doubt that there could be problems with thermal runaway and why should it happen with 6C33C?

On the subject of OTL amps I think it is important to choose a amp that is designed well, meaning that idle dissipation must be much lower than allowed maximum, that during normal operating conditions average cathode current should not exceed max allowed cathode current, that if parallell tubes are used current is derated, and that ventilation is good enough to remove the heat. However it seems that very few commercial amps fulfil these very basic requirements.

Regards Hans

[lousymusician]

Have you looked at these? I've never heard nor seen one, but they look promising.

[Spasticteapot]

Anything with 16+ tubes is going to have a failuire in at least one tube on a regular basis. Make sure to get a tube tester and a few backups while you're at it!

As a side note, you might want to consider trying the SEWA solid-state amplifier found in the Pass Labs section. It's supposed to be a pretty good 7 watt amp, and if you already have an appropriate transformer/PSU components, the cost is very low. (It's also supposed to work very well with a tube preamp - it has no gain!)

[Original Burnedfingers]

What does the impedance curve of those Cornwalls look like?

SY,

The speakers are too heavy for me to take into the shop to plot an exact curve but I did make impedance measurements with my impedance meter that will give a rough idea.

At 100HZ 5.0 ohms
330HZ 6.2 ohms
1K 9.2 ohms
10K 8.5 ohms


I tried to attach a response graph but it wouldn't take it.

[sorenj07]

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