What qualifies a "bad" tube amp, or what do you get from a more expensive tube amp

[bbgma1234]

My understanding is that the transformer quality makes a big difference.

when It comes to tubes versus solid state, four things are universal
1) there should be zero crosstalk between the left and right channels
2) the volume matching should be perfect between the left and right channels
3) power supply (60Hz) noise is not beneficial
4) it should work reliably

So the best tube amps are better in these elements.

Then you get into a lot of hand waving between good and bad distortion or non linearities. Ken Rockwell, the camera guy, has a couple of great comparisons on tube rolling and measurement results.

Bingo.
Also self-biasing

 

[valve_integrator]

You can and I have put tubes on a curve tracer.

The concern is that the objective measured quality and tube to tube consistency of peak tube makers of the 1950s and 1960s is not present in current tube manufacturers.

Using tube rectifiers is nonsensical.

several fallacies above.
I have large sample quantities of valves from the 60s and 70s, some of the best of the best UK prod.
"tube consistency" in my GEC sample of several 100s of mil spec (no less) is all over the show.

Emission in brand new devices (idle current) at identical bias is from below simple to double or more...the same was true of my large sample of older design STC 807 clones, while some even arced on startup.

The main reason why many valve manufacturers ran into QC trouble is because of legislation forbidding use of lead in glass, and some other polluting substances - no suprise they set up in places like PRC or Russia where even asbestos is de-rigeur - and smoking widespread.

Secondly
Valve rectifiers were used in TVs for years. (damper service)
They were designed to stand PIV of sometimes exceeding 5kv and up to 1/2 amp with new high quality cathode coatings.
If you use TV rectifiers they are by no means nonsensical, and give me an excellent PSU noise rejection, nice gentle HT ramp up and low voltage losses.

I use them in my amp, and they are unbeatable,- have only ever blown ONE up.

• Diode Peak Inverse Plate Voltage: 5kV
• Diode Peak Plate Current: 1.2A
• special quality heater material designed to stand very large cathode-heater voltage diffs. +300v / -5Kv.

 

[robwpdx]

several fallacies above.
I have large sample quantities of valves from the 60s and 70s, some of the best of the best UK prod.
"tube consistency" in my GEC sample of several 100s of mil spec (no less) is all over the show.

Emission in brand new devices (idle current) at identical bias is from below simple to double or more...the same was true of my large sample of older design STC 807 clones, while some even arced on startup.

The main reason why many valve manufacturers ran into QC trouble is because of legislation forbidding use of lead in glass, and some other polluting substances - no suprise they set up in places like PRC or Russia where even asbestos is de-rigeur - and smoking widespread.

Secondly
Valve rectifiers were used in TVs for years. (damper service)
They were designed to stand PIV of sometimes exceeding 5kv and up to 1/2 amp with new high quality cathode coatings.
If you use TV rectifiers they are by no means nonsensical, and give me an excellent PSU noise rejection, nice gentle HT ramp up and low voltage losses.

I use them in my amp, and they are unbeatable,- have only ever blown ONE up.

• Diode Peak Inverse Plate Voltage: 5kV
• Diode Peak Plate Current: 1.2A
• special quality heater material designed to stand very large cathode-heater voltage diffs. +300v / -5Kv.

Silicon diodes and now silicon carbide diodes can exceed those specs. There are tube socket pin compatible solid state rectifiers people commonly use in old guitar amps.

It would be hard to believe a tube rectifier can improve the measured audio specs of a preamp or amp. The imperfections in tube rectifier power supply design, like the sag, can contribute distortion to guitar amps for artistic reasons.

Tube or solid state rectifiers, the power supply mains transformer is going to produce roughly your plate voltage AC which will create the attendant high voltage electrical fields inside the chassis where low voltage signals are found. Tubes did their job in their time. Even today are still found in some high power RF amplifiers. Many compromises to linearity were made of necessity in the heyday of tubes in audio. Distortion measuring equipment always lagged audio tube amps. A to D converters, microprocessors, and computing FFT changed all that.

Tubes are superior in producing heat to linear solid state amps. Both tubes and solid state amps are superior to class-d amps in producing heat.

If people want to continue to use tube rectifiers, it is their choice.

 

[Mart68]

If you like that sort of thing, I doubt anyone here has much to say. I mean, I like anchovies on pizza

The local Pizza Hut stopped offering anchovies as a topping. I asked why and they said it was because I was the only customer who ever asked for them.

 

[valve_integrator]

Silicon diodes and now silicon carbide diodes can exceed those specs. There are tube socket pin compatible solid state rectifiers people commonly use in old guitar amps.*

It would be hard to believe a tube rectifier can improve the measured audio specs of a preamp or amp. The imperfections in tube rectifier power supply design, like the sag, can contribute distortion to guitar amps for artistic reasons.

Tube or solid state rectifiers, the power supply mains transformer is going to produce roughly your plate voltage AC which will create the attendant high voltage electrical fields inside the chassis where low voltage signals are found.

If people want to continue to use tube rectifiers, it is their choice.

Seems people like repeating the same old nonsense and assumptions.

I have both guitar and hifi amps.
The "guitar amp" was an old Bogen PA unit which is more than 70yrs old which I upgraded both to 230V op with a vintage British EI transformer and the whole PSU. (made in 1949 btw!).

It doesn't suffer significant SAG at all, mostly because it uses Choke input filter for the main HT, while I upgraded the valve rectifier for the Screen grid from a small one to a large one. (that makes a huge difference to the stability of the screen grid supply, which anyone at GEC or Mullard would always tell you is the secret of clean high power)

The main so called SAGGY HT supply starting through a silicon bridge is then fed to a valve rectifier as DC where both diodes in it are paralleled. That gives it a quiet HT supply with a nice surge free warm up.

As a result, the main HT (using a silicon bridge) is running over 630V instead of the original saggy 480-500.
The original dirty 50W output -was only clean-ish to about 25, now hammers out a genuine clean 75, and being AB2 gives a crazy dirty 130 when being driven into "clip". It's sort of a Jekyll and Hyde amp.
Everyone who has played on it says it kicks A so madly it makes any 100W Marshall or Mesa sound weak at the knees.

The secret? Getting the BIAS derived from a stable source, rather than the original resistor chain, and changing the original NFB loop, while using large 150V gas stabilisers for the screen supply - nothing to do with the stuff you wrote.

I have an identical pair of the HO50 of the same age which I also upgraded for home cinema use.
They use a completely unobtanium quad of KT8C - which are basically a KT66 with funny bases and top caps.
The prices for vintage KT66 are insane, but TBQH I couldn't tell any difference either measured or in listening tests against the original 807s from JAN stocks.
That encourages me to believe those inflated prices for original GEC KT66 for Radford and Quad vintage amps are all BS.
The similar successful recipe was used for the PSU and a lot of care taken with the NFB and Bias circuits.
Again, the screen grid supply is derived from a valve rectifier.
They make an ultra clean 60W RMS into any load.

They don't have that upwardly spiralling curve of distortion you get as power increases, characteristic of all commerical valve amps I have ever tested, but there again the Audiophile crowd with their fancy power conditioners and blingy speaker cables would be poop their pampers if you ran flat out tests for hours into one of their "hifi" amps, and have never heard of running control grids (g1) into heavy +ve territory.
(I'm 100% sure they would blow up within 2 -5 mins)

I am aware the sound these old classics make has odd harmonic distortion - contrary to all those "valve experts"
banging on about valve amps only making "nice" even harmonics.
It's probably the combo of AB2 operation and beam tetrodes that give it the sound it has.

FYI, if you look up the data for KT66 and KT88, you will find they are fairly non linear IMD machines, which is why of course no amp maker will ever give you IMD figures at anything over 1W and the standard SMPTE test.
(60 Hz and 7 kHz, with 4:1 amplitude ratios).
If I wanted to make them better I would change the output transfomers - but hey why bother?

Finally the compact hifi amp I designed, which is thoroughly modern, and took me several years of late nights to decide on.
The screen grid supply is rectified with a silicon bridge but which is then fed into a special quality triode HT stabiliser circuit, with gas discharge voltage reference. (Russian made SG5V).

That ensures there is zero supply noise going into the AF circuit while being able to supply a lot of current to the 8 screen grids...(pentodes instead of beam tetrodes - cutting down IMD this way to almost zero).
The screen and main HT supplies are fed from seperate windings on the toroid transfo, so they can't interact.

ie. The whole idea that valve rectifiers sag is basically bollox.
If you feed them with rectified DC instead of AC I get a drop from idle to full 100W RMS power - both channels driven of barely 20V on the damper diode. Feeding again a choke input filter, the HT supply is ultra stable.

The specially made250VA core toroid transformer itself gives more voltage drop than that, and has been the only item which has gone up in smoke. (twice in fact - ONCE because of lightning)
The unit in a 3U case is so light it can travel with me on an aircraft as accompanied baggage. (under 17kg)
It looks like a solid state amp, because all the valves are hidden - nothing can be seen to be glowing, and can be dropped heavily without smashing it to pieces. (thanks aeroflot - they did exactly that!)

It can be powered off a UPS.
Driven flat out, my APC UPS states the whole unit is using 250W - which is quite OK inside a car. I doubt very much you would ever want to run the 2 x 50W of valve amp power in a car - you would end up deaf!
250VA in, for 100W of very clean distortion free audio is in my book pretty OK, considering the heaters use 50W.
Again, distortion stays well below audible threshold to within 1-2dB of clip.

All listening tests we did with it confirmed everybody thinks it sounds fantastic, and it is stable into any load, inductive, capacitive or resistive.
The output transformers? Transcendar, - clones of 1963 SCOTT 208.
Absolutely brilliant - sad, Gerry who makes them went out of business.

 

[robwpdx]

ASR is fortunate to have engineers like @valve integrator who can make the best of tubes at a deep level of understanding. Better yet, a musician making artistic decisions on specific material. I'm more clean listening, live and reproduced.

I have spent a lot of time listening to MC240s and all manner of professional tube and transformer signal chains. I would not go back to them now, including tube rectifiers. The more tubes, reel-to-reel tape, and the more analog consoles you have, the more techs you need. I, like you, are my own tech. I'm an EE and I think the new digital tube curve tracers are cool, but I wouldn't buy one.

Do you want some 1950's Bogen T155 input transformers? I think I have 2, but I have to find them.

 

[valve_integrator]

ASR is fortunate to have engineers like @valve integrator who can make the best of tubes at a deep level of understanding. *

Better yet, a musician making artistic decisions on specific material. I'm more clean listening, live and reproduced.**

I have spent a lot of time listening to MC240s and all manner of professional tube and transformer signal chains....***

lots to comment on here.

****Do you want some 1950's Bogen T155 input transformers? I think I have 2, but I have to find them.

*very flattered. I do have references, eg. David Gillespie, - tronola.com, Jack Elliano of electraprint..
These people know what they are on about and we often agree to disagree.

** actually I am a pro musician + sound engineer. I record mostly live - NO STUDIOS.

*** dry, lifeless, sterile.
Terms often used to describe MC240, fairly typical of amps with excessive amounts of NFB..we know why!

DG above says the same everything has to be applied (like herbs and spices in good cooking) in modest careful amounts. Too much NFB is as bad as those freaks insisting on zero NFB.

I note:- MC240 a so called "audio legend "
Total harmonic distortion: 0.5% - not stated where - probably 1W at 1khz.

No quoted IMD figures.
6L6GC are dead reliable - with those aligned grids, and the larger anodes.
BUT - big but...
They are IMD producing machines.
GEC states:- KT66.....IMD 3% - in TRIODE connection, 4% in Ultralinear Cathode bias, and an amazing 15% in fixed bias (!)- ie. real filthy dirty.

IMD is the most objectionable form of distortion.
I have no doubt Quad and Radford had the same problems, but hey feed them with band limited variable speed always distorting vinyl and I'll bet you never know eh?
Bang them thru those "narrow beaming" Quad ESLs or those overrated overhyped BBC LS3/5a and it's back to the pink spectacles past with even a "BBC DIP" to make them sound authentic.

****I have broken up loads of old Bogens especially the old MO/MX series cos at 50-100USD it was a give away for the OPTs.
It also gave me stacks of the 7247 and 6EU7, (a low noise 6V 12AX7) + lots of nr new 12AX7
The MO/MX60/120 Bogen PA amps sounded pretty awful in STD form for anything other than speech because they ran them pretty much in class B.
The output transformers once run properly in Class A or AB, esp the huge MX60a model are brilliant, as are the very large MO100A/MO200.
U can run the 25V CT output winding as cathode feedback....althugh I appear to be the only person that has ever done it, as well as subbing in the excellent 6G-B8s instead....

I am not short of the input transfomers - which for some reason lots of people seem to want to buy.

 

[valve_integrator]

Only a little further remark.
I reckon the MC 225 was probably the best they made.
Westinghouse designs were good.
The 7591 was a 1/2 sized 8417 - so read on.

I have never heard one of these but the Fisher Sa-1000 should be a candidate for an ASR vintage test.
I have a lot of admiration for Avery Fisher.
He did a huge amount for musicians and music, which none of the current generation would come ever close to.

Does that help qualify a top of the range valve amp compared with all those PRC made junk?