[Roger A. Modjeski]

Hi,

My apologizes for being away from my circle for awhile. I have been intensely studying a number of things to improve our listening pleasure. Here's the list.

1. I've made some considerable improvements in transformers that I wind myself and have made them available on the RM-200. Check this out.   http://ramlabs-musicreference.com/pricelist.html

2. I've been studying bias drift in output tubes. See post on that topic.

3. I've been looking into amplifier stability. My amps are more stable than most out there and I'm going further into this issue. I'll bet anyone $100 I can make their "Unconditionally Stable Amplifier" oscillate. See my post on "Birdies"

4. Considering an OTL amp. Any suggestions on power and impedance?

5. Studying some Horizontal output tubes for a line of OTL and Transformer coupled amps. Is that plate cap going to bother anyone?

6. I am winding some special transformers to convert a RM-10 to dedicated mono with increased power into 8 and 4 ohms. There will be a new, larger output transformer under the hood.


I welcome your comments and promise to figure them in but will not be able to answer many questions as I have a lot of winding to do.

[mgalusha]

I love the sound of OTL amps and will follow with great interest in your development of this. Power and impedance, with OTL's the seem pretty closely linked since it usually takes paralleling lots of output tubes to get the impedance down fairly low without a ton of feedback.

As for plate caps, they don't bother me as we have no children in the house and the system won't be turned on if the grand kids are around.

[BobRex]

I second the OTL idea, maybe something in the 10-20 watt range to start.  I've been contemplating changing out my Moondogs for a little more power and both the Transcendent T16 and Atma S30 are on my short list, so I'd welcome another OTL candidate.

[DustyC]

Regarding no. 5, the plate cap doesn't bother me either. With those sweep tubes you get a lot of power from a minimal tube count. (150 watts?) And those sweep tubes are cheap too!

[kmanusa]

One note of caution-- I owned a Melos 200 Triode amp that used 6KG6 horizontal sweep tubes.  These tubes were not reliable in this circuit, nor were they readily available.  I would hate to see Music Reference build an amp using sweep tubes that was not reliable. 

There were some novel design ideas in the Melos equipment but I had lots of reliability problems with this amp beyond the tubes and the company is out of business.  I know Roger would not market a product that is unreliable so perhaps I am beating a dead horse.  But, I spent a lot of time and money replacing 6KG6 tubes, as well their protective screen grid fuses, in that beast.  And the only source for the 6KG6 at that time was direct from overseas.

My feeling about sweep tubes is perhaps biased by my Ham radio experience with them as used in HF radio transmitter finals.  I prefer "transmitting tubes" (e.g., tubes designed for that application such as the 6146) to sweep tubes in radio transmitter final amps because they are more rugged.  I suspect tubes like the 6550 are more rugged than sweep tubes in high power audio applications too but I am speculating here, perhaps Roger would like to weigh-in on this subject?

[tybee]

Sorry to interrupt possble new amp designs, but I wish Rodger would offer a phono preamp for the SES series.  I just picked up a C4 preamp with tone controls from another AC member to match my EM7 amp. What a great sonic combo.  I have been listening to LPs all morning, and the SES series works extremely well. Wish I had a MR SES phono pre to match, Roger?

[Roger A. Modjeski]

One note of caution-- I owned a Melos 200 Triode amp that used 6KG6 horizontal sweep tubes.  These tubes were not reliable in this circuit, nor were they readily available.  I would hate to see Music Reference build an amp using sweep tubes that was not reliable. 

There were some novel design ideas in the Melos equipment but I had lots of reliability problems with this amp beyond the tubes and the company is out of business.  I know Roger would not market a product that is unreliable so perhaps I am beating a dead horse.  But, I spent a lot of time and money replacing 6KG6 tubes, as well their protective screen grid fuses, in that beast.  And the only source for the 6KG6 at that time was direct from overseas.

My feeling about sweep tubes is perhaps biased by my Ham radio experience with them as used in HF radio transmitter finals.  I prefer "transmitting tubes" (e.g., tubes designed for that application such as the 6146) to sweep tubes in radio transmitter final amps because they are more rugged.  I suspect tubes like the 6550 are more rugged than sweep tubes in high power audio applications too but I am speculating here, perhaps Roger would like to weigh-in on this subject?

Sweep tubes are quite reliable when properly applied. There are no recommended audio applications for these tubes so the performance of the amplifier is entirely dependent on the knowledge of the fellow engineering the amp. I use the term engineering here because it takes one that understands what tube parameters can be changed and still keep the tube happy. Tim de Paravicini, Julius Futterman and I have made good sounding amplifiers employing sweep tubes with years of proven reliability.

I have not seen the circuit or operating paramaters of the Melos 200 but here are a lot of hits on the net, many of which deal with problems. Is Melos still around?

I will say that using sweep tubes in triode mode is a very bad idea and I can't imagine an amplifier having any significant output power per pair of tubes. How many watts per pair is he getting? What is the plate voltage and bias current?

The 6KG6 has gone from plentiful to rare in the past year. Up till then it was very cheap and easy to get. So much so that modern ham radio and illegal CB linear amps were made with them. It's the radio guys who used them all up. When the USA decided to block trade with Yugoslavia due to the conflict there were some 18,000 6KG6 tubes impounded on the tarmack at JFK airport. They were locked up for about 8 years. I bid on them but the seller wanted to sell the whole lot to one person.  There were a lot of unusable types in the lot because the EI factory dumped their dead inventory on their fellow countrymen doing the importing in NYC. There's a lot more to this story, but that's the part that concerns these tubes.

For those needing 6KG6 tubes I have only a few but I do have a good supply of Phillips/Sylvania 40KG6 that can be used in the same socket by changing the filament voltge.

[kmanusa]

I have not seen the circuit or operating paramaters of the Melos 200 but here are a lot of hits on the net, many of which deal with problems. Is Melos still around?

I will say that using sweep tubes in triode mode is a very bad idea and I can't imagine an amplifier having any significant output power per pair of tubes. How many watts per pair is he getting? What is the plate voltage and bias current?

The 6KG6 has gone from plentiful to rare in the past year. Up till then it was very cheap and easy to get. So much so that modern ham radio and illegal CB linear amps were made with them. It's the radio guys who used them all up. When the USA decided to block trade with Yugoslavia due to the conflict there were some 18,000 6KG6 tubes impounded on the tarmack at JFK airport. They were locked up for about 8 years. I bid on them but the seller wanted to sell the whole lot to one person.  There were a lot of unusable types in the lot because the EI factory dumped their dead inventory on their fellow countrymen doing the importing in NYC. There's a lot more to this story, but that's the part that concerns these tubes.

For those needing 6KG6 tubes I have only a few but I do have a good supply of Phillips/Sylvania 40KG6 that can be used in the same socket by changing the filament voltge.

I never saw a schematic, that was part of the problem with the Melos stuff.  There are no schematics to be had and they went out of business several years ago.

He claimed 200 watts per channel with the 200 Triode Plus using 2 6KG6s per channel.  He was using "screen drive" but with no schematic, it was hard to tell what he was doing with the circuit.  He used an IC to perform auto-bias with LED indicators.  I had problems with noisy, defective capacitors and output tubes blowing.  I fixed the problems the best I could w/o a schematic then sold the amp at a sizeable loss.  I provided a NOS set of Amperex 6KG6s to the buyer.  I had an inkling what I was getting into with the thing when I bought it so there are no hard feelings.

Glad to see folks have been successful with sweep tubes properly engineered for the application!  Your story about the Yugo tubes is fascinating!  I would bet there is some scammer who is dying to get his hands on that load of Yugos to sell to the illegal linear crowd!

[DARTH AUDIO]

OTL sounds interesting. How about 150 to 200 watts per side?

[Roger A. Modjeski]

I plan to make several different power levels. Power output is very impedance dependent, the higher the better. The original Futterman amps were made to run 16 ohms. The SA-4 had 8 output tubes and achieved 60 watts into 4 ohms,  100 watts into 8 ohms and 150 into 16.

One can improve the power output into lower impedance speakers with a Autoformer having taps at 4,8,16 ohms. A tranformer for these low voltages is smaller and has much better response than a standard output transformer that has to deal with high plate voltages. Furthermore, the autoformer will be out of the feedback loop and can actually increase stability. I have that part done already.

[6BQ5]

Roger, have you ever given any thought on developing a full-function integrated amplifier, which marries the RM-5 IV and RM-10 II in one box?

This would probably not take up much time (a winter project??) to complete given you've got these two proven designs under your belt. In fact, you don't need all that gain in the preamp RM5 ... you can go passive with one of your passive designs except for the phono stage (which is a must IMO).

Not sure if you are familiar with a Japanese brand called Leben, but they've had good success selling their (retro looking) CS-300 integrated amp which uses EL84s as output tubes. What would make it sweet still, is if you can add a headphone jack and a bass contour control that you've mentioned too on these forums. I've been tempted by this little Leben package ... though continue to remain loyal to my MR gear. With the way the Yen has shot up of late too, this little amp has become very expensive piece of kit.

http://www.6moons.com/audioreviews/leben/cs300x.html

[Russell Dawkins]

Speaking of unusual-but-useful controls - I'd like a "tilt" control, like Quad used to put on their control units. It does just what the name suggests - tilts the response with 1kHz as a fulcrum. Their illustration (probably a little fanciful) showed the result as a straight line, tilted down or up, passing through 1,000 Hz.

[Thirsty]

Looks like I'm a bit late in responding to this thread but here's another vote for an OTL, I think it would be a great idea to include the autoformers built into the amp for those of us running lower imp. speakers. I currently run a pair of 2a3 mono's on my 98db  8 ohm
Klipsch's and although I never seem to run out of steam I'm thinking a few more watts for the added headroom would be a good idea, so a low powered OTL of around 15 watts or so should be plenty.

[Roger A. Modjeski]

Happy to hear your suggestion for the internal matching transformers. I plan to make them both ways. The matching transformer can be external in a nice box at the speaker allowing a high impedance run to the speaker. It is outside the feedback loop which provides speaker protection and better amplifier stability.

For those curious about adding a matching transformer to an OTL, which seems silly, it actually has several advantages. Consider that the matching transformer is designed to present the ideal load to the OTL output stage. This may be as high as 64 ohms, for which  no speakers exist. This matching transformer is an autoformer with a low turns ratio of 2 -4 to 1. Because there are no high voltages present and no bias current running in the transformer it can be wound in such a way that it has very wide bandwidth and is smaller than a standard output transformer of the same power rating.

[EDS_]

Roger, have you ever given any thought on developing a full-function integrated amplifier, which marries the RM-5 IV and RM-10 II in one box?

This would probably not take up much time (a winter project??) to complete given you've got these two proven designs under your belt. In fact, you don't need all that gain in the preamp RM5 ... you can go passive with one of your passive designs except for the phono stage (which is a must IMO).

Not sure if you are familiar with a Japanese brand called Leben, but they've had good success selling their (retro looking) CS-300 integrated amp which uses EL84s as output tubes. What would make it sweet still, is if you can add a headphone jack and a bass contour control that you've mentioned too on these forums. I've been tempted by this little Leben package ... though continue to remain loyal to my MR gear. With the way the Yen has shot up of late too, this little amp has become very expensive piece of kit.

http://www.6moons.com/audioreviews/leben/cs300x.html

I'd be very interested in an integrated as well.

[zybar]

Happy to hear your suggestion for the internal matching transformers. I plan to make them both ways. The matching transformer can be external in a nice box at the speaker allowing a high impedance run to the speaker. It is outside the feedback loop which provides speaker protection and better amplifier stability.

For those curious about adding a matching transformer to an OTL, which seems silly, it actually has several advantages. Consider that the matching transformer is designed to present the ideal load to the OTL output stage. This may be as high as 64 ohms, for which  no speakers exist. This matching transformer is an autoformer with a low turns ratio of 2 -4 to 1. Because there are no high voltages present and no bias current running in the transformer it can be wound in such a way that it has very wide bandwidth and is smaller than a standard output transformer of the same power rating.

This isn't silly at all Roger.

I use a pair of Paul Speltz Zeros (http://www.zeroimpedance.com/Zero-New.html) to pretty much do what you are suggesting.

George

[cujobob]

My mcalister pp150 uses sweep tubes and sounds great but is not the most detailed amp around.  I'd love to see some alternatives.