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Brit01 7th July 2009 12:21 AM

Futterman style 6AS7 OTL with other tubes
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Can the Futterman style design for the 6AS7 be modified to use other tubes? if so what types?

What is the plate impedance of the 6N6P? 68 ohm? Is the symbol Ri for plate impedance?

I like the simplicity of the Futterman design and would be interested in experimenting.

kevinkr 7th July 2009 04:00 AM

Based on prior experience I would say your +/- 60V supplies are a bit too low, something on the order of +/- 100V - 120V is more likely the case. The Russian made equivalent of the 6AS7 works very well in OTL amps.

The symbol for plate resistance is rp, but don't be misled as there are other losses in the tube that are not directly reflected by the plate resistance such as limited emission, etc., that come into play particularly at high plate currents.

The 6C33, 6336A/B/C are also good choices and a single one of either will comfortably replace a pair of 6AS7..

Rather than substituting different types in this design I would build the basic design, learn how it works and then perhaps build another using more exotic types.

Note that I would not necessarily recommend an OTL as a first tube project which I suspect this might be. They look simple on the surface but getting them reliable and safe is an arcane art not easily mastered by a newbie.

The output stages require the same sort of power supply capability current wise as a solid state amplifier, but at much higher voltages to account for the voltage drops in the output tubes under load. Several amperes can be required for reasonable output power levels and it has to be well filtered as well because the design has mediocre PSRR - this gets very expensive.

Building a proven design as intended is a much safer bet.

Brit01 7th July 2009 12:19 PM

Hi Kevin,

Thks for the info. Yes 60V did seem awfully low for the plate voltages in this design.

It will be my third tube project. I have built one liestage and one headphone amp up to date now but yes relatively a newbie you could say.

I was considering this type of amp due to the non-existent supply of OPT's here in Uruguay and mission impossible of importing any.
Cuba like customs:bawling: :bawling: :bawling:
Custom built is the only option, I think there is only one guy in this country who 'may' be able to construct an OPT but I doubt he's had much experience of this. I know he can build good power supply trannies though.

Let me read further into this. Wish there were more hours in the day.

HollowState 7th July 2009 05:34 PM

Re: Futterman style 6AS7 OTL with other tubes

Originally posted by Brit01
Can the Futterman style design for the 6AS7 be modified to use other tubes? if so what types?
Sweep tubes were used by old man Futterman near the end in the OTL 1 and OTL 3. They are very complicated designs and I'm told they could be "oscillating devils", perhaps because of the gain in the tubes. They were manufactured on PC boards, not hand wired. I would not recommend these even to experienced builders. You can find the schematics at the link below.

Brit01 8th July 2009 05:26 PM


Maybe a dumb question awaiting a very good answer:

If getting impedance down to a low level has always been a challenge in these OTL designs would it be feasible to build one of these amps to drive a line array speaker with small speakers wired in series to create a high impedance for the amp?

SY 8th July 2009 05:53 PM

Yes, that's an excellent application. High load impedance is Good.

Brit01 8th July 2009 06:41 PM

Then this sounds like an interesting project.

A whole bunch of cheap small cones as a line array project wired up in series to match the impedence of an OTL.

Thinking about small 3 or 4 inch speakers in a slim mdf case.

10 x 8 ohm speakers at 80 ohms in total.
OTL designed for 80 ohms per channel without feedback.

SY 8th July 2009 06:43 PM

GMTA. I have about 50 drivers OEMed for the NHT Super Zero just waiting for me to get a free weekend to cut some wood. My plan is to use a 6528-based Circlotron.

bbp 8th July 2009 07:16 PM

The 6N5P 6N13P made in China will work fine,the two tube only cost about $2 / pcs
6C45PI is another choice

HollowState 8th July 2009 08:31 PM

Making an OLT amplifier is a project I've had on the back burner for quite a while as I have plenty of suitable tubes on hand. Just no free time in quanity. These include 6528s, 6336A/Bs, 6394s, (26V 6336A) and some Bendix carbon plate 6082s (26V 6080).

As long as the amplifier's output remains balanced, all is fine. But any OTL amplifier, be it Circlotron or totem pole, exposes ones speakers to the risk of high DC voltage should a tube fail or short and offset the output. I personally consider Chinese tubes to be the least desirable in terms of reliability and trustworthiness. (sorry about that)

For this reason, unless you have cheap speakers that can be easily replaced, I would consider some form of fail-safe protection circuitry. Fusing the output at minimum. Or perhaps monitoring the current of each output tube (across a small resistor) with a comparator IC driving a transistor stage(s) to activate (or deactivate) a relay in series with the speaker output. Just food for thought.

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