GM70 tube copper vs graphite plate???

[kevinkr]

I would not make the mistake of generalizing from one tube type to another. Let's confine this to what we know wrt the GM70.

Also when people post comments about which tube they prefer I think the context in which they are used is helpful.

For example when I posted I overall preferred the graphite version I also noted the load impedance was 7K, plate voltage is 1kV and operating current is 110 - 120mA. No negative feedback is used in either stage. Driver tube is triode connected D3A IT coupled to the GM70. Fixed bias is used in both stages. Speakers are Onken based with 16" Iconic 165-8G for woofer and 16 ohm 2440 and 075 JBL horns on the top.

The graphite in my limited experience has plenty of top end sparkle in my system and the bass is much better. (My system reproduces down to 35Hz) The copper seems subjectively very slightly more refined in the mids and highs, but has other issues that make it my second choice. I initially thought I liked them better but long periods of comparative listening leaves me convinced that almost all of the subjective tonal differences may relate to nothing more than the additional energy on the bottom end with the graphite plate version - the graphite is definitely more dynamic sounding and I think a bit more convincing than the copper in my system.

I am talking of subtleties here, either should be more than satisfactory in any properly designed amp.

 

[palmas]

I would not make the mistake of generalizing from one tube type to another. Let's confine this to what we know wrt the GM70.

Also when people post comments about which tube they prefer I think the context in which they are used is helpful.

For example when I posted I overall preferred the graphite version I also noted the load impedance was 7K, plate voltage is 1kV and operating current is 110 - 120mA. No negative feedback is used in either stage. Driver tube is triode connected D3A IT coupled to the GM70. Fixed bias is used in both stages. Speakers are Onken based with 16" Iconic 165-8G for woofer and 16 ohm 2440 and 075 JBL horns on the top.

The graphite in my limited experience has plenty of top end sparkle in my system and the bass is much better. (My system reproduces down to 35Hz) The copper seems subjectively very slightly more refined in the mids and highs, but has other issues that make it my second choice. I initially thought I liked them better but long periods of comparative listening leaves me convinced that almost all of the subjective tonal differences may relate to nothing more than the additional energy on the bottom end with the graphite plate version - the graphite is definitely more dynamic sounding and I think a bit more convincing than the copper in my system.

I am talking of subtleties here, either should be more than satisfactory in any properly designed amp.

Agreed. I use the GM70 with 10k load (10k/4) and the driver is a 300B.

 

[djn]

Well, there sure is a lot to think about here. Thanks for all the good dialogue. I use this amp to drive my 350hz wood tractrix horns with JBL 2441 CDs. I have the Big A$$ Heils for tweets and Lambda TD15X for woofs and four subwoofers.

I should have stated this right up front as I am looking for some understanding to the sound from 700hz to 7000hz.

 

[kevinkr]

Either type should suit this application, and best of all neither kind are going to set you back much money so I would recommend you try both and decide based on your own experience.

So far I have had an excellent experience acquiring my GM70s on eBay, unfortunately the situation in the Ukraine may somewhat limit your choice of seller. I have had good experiences with both Russian and Ukrainian sellers on eBay, must be cultural but these are some of the best and most honest sellers I have encountered on eBay.

 

[djn]

Thanks K. I have graphite ones already, but the copper ones are going for $100 a tube. That is a lot of money for just a little bit of sound quality increase, but not so much if it is day and night.

 

[kevinkr]

The way I look at it is that they are all very inexpensive compared to the 300Bs I used to use. I guess it is all frame of reference.

It's not day and night in my application, but I cannot claim one way or the other for yours. Anyone nearby you might be able to borrow a pair from? That's how I actually got my first pair of coppers, bought them subsequently and them bought some on eBay.

 

[JMFahey]

I have both and have been using both (1200V/100mA SE).
I prefer the copper version, the sound is more open and detailed. The carbon M70 is more bass heavy but misses the top-end sparkle.

So the copper plate one has wider frequency response (within the audio band) than the graphite plate one?

Weird, they are both RF tubes

Did anybody trace the frequency response of both so they can be compared?

There should be at least a couple dB difference between them if any difference is so marked.

 

[esltransformer]

And maybe including which transformer is used.
Using transformers which has -3dB at 35kHz isn't helping.

I would not make the mistake of generalizing from one tube type to another. Let's confine this to what we know wrt the GM70.

Also when people post comments about which tube they prefer I think the context in which they are used is helpful.

For example when I posted I overall preferred the graphite version I also noted the load impedance was 7K, plate voltage is 1kV and operating current is 110 - 120mA. No negative feedback is used in either stage. Driver tube is triode connected D3A IT coupled to the GM70. Fixed bias is used in both stages. Speakers are Onken based with 16" Iconic 165-8G for woofer and 16 ohm 2440 and 075 JBL horns on the top.

The graphite in my limited experience has plenty of top end sparkle in my system and the bass is much better. (My system reproduces down to 35Hz) The copper seems subjectively very slightly more refined in the mids and highs, but has other issues that make it my second choice. I initially thought I liked them better but long periods of comparative listening leaves me convinced that almost all of the subjective tonal differences may relate to nothing more than the additional energy on the bottom end with the graphite plate version - the graphite is definitely more dynamic sounding and I think a bit more convincing than the copper in my system.

I am talking of subtleties here, either should be more than satisfactory in any properly designed amp.

 

[kimbal]

I had the misfortune of receiving four fairly rare Souz graphite plate GM70 from Ukraine that were improperly packed, internal structure and filaments completely destroyed. The seller made good and replaced them with comparables of another vintage and manufacture. I was really looking forward to trying those.

Most of my graphites seem to have been made by Reflektor.
The coppers were all made by different makers, but I am not sure who made any of them.

I feel for you with the broken GM70 tubes.

I had a similar situation back in 2003 with 5 out of 7 - 211 tubes - being poorly packed from the USA and all had broken Filaments when they arrived inside their original boxes some weeks later.
I got them off Ebay and no one took responsibility for the breakage - sender, who ignorantly was selling for a deceased Ham Radio estate and had no idea what they were doing with tubes, the post office and of course good old Ebay.

I lost about $1000 dollars out of the exercise with a lot of heart ache and tears. Hence I am very cautious about buying tubes off eBay - PERIOD !

I could be a real prick and resell them on eBay - but I have principles - unlike some people. So Buyer - Beware !

 

[palmas]

So the copper plate one has wider frequency response (within the audio band) than the graphite plate one?

Weird, they are both RF tubes

Did anybody trace the frequency response of both so they can be compared?

There should be at least a couple dB difference between them if any difference is so marked.

It is something you hear but not something enough to measure. These tubes go from DC to high frequency (transmiter tubes).

Use the carbon type and when you get the chance, try a pair of copper and hear what you prefer.

 

[kevinkr]

That nomenclature wrt RF triode in the English language datasheets is probably not entirely correct. I have seen others in the past that clearly stated it is Low Frequency Power Triode. They were commonly used in large PA amplifiers in the 500Wrms and 1KW power class found in stadiums and other large venues. They may have been used in some RF applications but I am not aware of any.

Strangely I can no longer find any of this stuff on Google so take my comments with a grain of salt.

 

[kevinkr]

So the copper plate one has wider frequency response (within the audio band) than the graphite plate one?

Weird, they are both RF tubes

Did anybody trace the frequency response of both so they can be compared?

There should be at least a couple dB difference between them if any difference is so marked.

See previous post - they were designed as low frequency triodes, not aware of any RF applications for this tube. No longer can find the original amplifiers these were designed for on the internet either.. (Odd)

I have done casual measurements into load resistors and I can't recall measuring any difference at all in frequency response. Any significant difference would have set off alarm bells in my head. The response in my design is entirely transformer limited as ESL points out.

 

[JMFahey]

It is something you hear but not something enough to measure. These tubes go from DC to high frequency (transmiter tubes).

Use the carbon type and when you get the chance, try a pair of copper and hear what you prefer.

Sorry but if you hear such a marked difference in (audio) high frequencies that means a very measurable difference.
A measurement microphone is much flatter, repeatable , etc than anybody´s ears.

As in, for example, it can measure beyond 20KHz, have a calibrated response, give you an exact SPL level (try that by ear) , etc.

Now if 2 tubes give you exact same measured response, yet you "hear" one as brighter, the other duller .... who do I trust?
Just sayin'

What you are pointing at is not some mystery subjective quality such as soundstage, Prat, Musicality, whatever, but some very defined and measurable one.

 

[palmas]

Sorry but if you hear such a marked difference in (audio) high frequencies that means a very measurable difference.
A measurement microphone is much flatter, repeatable , etc than anybody´s ears.

As in, for example, it can measure beyond 20KHz, have a calibrated response, give you an exact SPL level (try that by ear) , etc.

Now if 2 tubes give you exact same measured response, yet you "hear" one as brighter, the other duller .... who do I trust?
Just sayin'

What you are pointing at is not some mystery subjective quality such as soundstage, Prat, Musicality, whatever, but some very defined and measurable one.

Sorry, could not measure any marked difference, but yes, I and other users can clearly hear it.

 

[palmas]

See previous post - they were designed as low frequency triodes, not aware of any RF applications for this tube. No longer can find the original amplifiers these were designed for on the internet either.. (Odd)

I believe the GM70 were used as transmiter tubes in some russian tanks

 

[kevinkr]

Way too fragile for transmitter use in something like a tank. Improperly packed GM70s can be destroyed in transit through the mails - something I have direct experience of. The 6C33 was used as a pass element in power supplies in some MIG fighters, but that is a much more robust device.

 

[kevinkr]

Sorry, could not measure any marked difference, but yes, I and other users can clearly hear it.

There may be a very small wideband deviation in response due to a difference in rp - which is something I can believe, and in addition I can also believe there may be slight differences in distortion spectrum as well. I can attest to hearing subtle differences between the two, but I prefer the graphite.

My measurements were very casual, I no longer have the data, and they were not simultaneously executed so I cannot rule out the possibility that there are small measurable differences that could be audible. Nothing leapt out at me as being particularly different between one type and the other at the time.

I've no plans to remeasure as the amps have been in service for nearly 3yrs now without issues; I am satisfied with both their electrical and subjective performance.

 

[esltransformer]

Maybe there is a small differance in Rp but maybe also in the capacities.
All will be influence the frequenty response but the most influence has the loudspeaker.
Without loudspeaker the transformer has the most influence.

Measurements are importent but what this measurement excactly means is sometimes very difficult. Frequency respons must be wide, no resonance! No resonance outside the -3dB area!

Ik know a lot of well known transformer manufactues don't give frequencies curves and square wave response of there transformers. Maybe because it's not as good as it shoud be......

There may be a very small wideband deviation in response due to a difference in rp - which is something I can believe, and in addition I can also believe there may be slight differences in distortion spectrum as well. I can attest to hearing subtle differences between the two, but I prefer the graphite.

My measurements were very casual, I no longer have the data, and they were not simultaneously executed so I cannot rule out the possibility that there are small measurable differences that could be audible. Nothing leapt out at me as being particularly different between one type and the other at the time.

I've no plans to remeasure as the amps have been in service for nearly 3yrs now without issues; I am satisfied with both their electrical and subjective performance.