70s SVT Bench Test Results

[bobyoung53]

It was reported that there was a problem with the 6146B's plate dissipating the heat. That is one reason why they switched to the 6550.

I know it but I think that was when they were supposedly constantly cranked up to 11 by The Stones. I know the 6146 family of tubes are not really reliable, I have a transmitter that uses them and I use 6293's instead the can handle a lot more sustained power. Those will sub for the older 6146's though I don't know about the 6146B's, I know the 6146B's not really interchangeable with the 6146's if they were I'd use the 6293's as subs for the 6146B's in an SVT.

 

[JMacBass65]

Probably not much other than knowledge for knowledge's sake. I can totally understand people not finding it useful or even interesting.

Well, I am reading all of it, because I own a 1976 SVT, so I am trying to learn..........something.

Yes... You have a valid point...
The 6146B big claim to fame was the "Dark Heater" ...this allowed the heater to maintain an adequate space charge durring lower heater voltage operation...
If you take a look at the tiny wire gauge used in the SVT for the heater wires, it's ridiculous.... The drop across the heater line is insane... starting at output tube 1 you may be at 6.3V but your are 5.7V by the time your at output tube #6 .. And thats at IDLE !! LOL..
At full power output the heaters will dip down a lot more....
Even though the heaters are on a different transformer...The 120V at you Mains or Variac will droop significantly from the long AC cord then the small wires that feed the 120V to FUSE holder then to the MOLEX connector... up to the pre-amp..through more tiny wires and ON switch.. then back down through the MOLEX then through some more tiny wires, then to the transformer primary leads...LOL if you think there is 120V entering the transformers at this point...
I have previously mentioned I use a relay for the 120V ...and do not pass all that current up through the MOLEX and switches... I install a relay in the socket location of the where the delay tube would have been..
I also use HUGE wire on the heaters across the tube sockets.....also re-wire so that the heater feed is at the center of the 6 sockets..ie between socket #3 and socket#4 ...

What in the actual hell was all of that about??

You guys are all WAY smarter than me.

 

[beans-on-toast]

I misspoke, I meant the 6.3 volts on the heaters, not the cathodes. Regardless of the biasing method, the emission of the cathode is dependent upon the temperature of the heater. The specs for tubes show a marked performance decrease with slightly low heater voltage and premature wear and heater burnout with higher voltage. Both conditions affect cathode emission, in preamp tubes and driver/splitters in addition to power tubes. In my 60's.tube amps, designed to run on 117V but frequently at 125V, I have almost 7 Volts on the heater string. As this is a reality of my local utility, I have installed resistors and diodes in my vintage amps to drop the heater voltage back into range. Those having a similar situation might be able to afford the cost and weight of a well regulated UPS, or are using commie tubes that are easily and cheaply replacable, but I use only US and western euro vintage tubes that are too expensive to sacrifice. YMMV.

The 6146B tube data sheet talks about heater voltage. They state that for a system that needs to be on standy for periods of 15 minutes to 2 hours, they recommend reducing the heater voltage to 80% of normal; beyond 2 hours, shut the amp down. All this is in aid of longer tube life.

Line voltage keeps creeping up in North America and in some locations can be 125 and higher. Although amps are rated to run over a range of voltages, it doesn't mean that operating the amp at the upper limit isn't hard on it. It can affect the long term service life of capacitors and tubes.

If an amp is designed to run on 120vac and the line voltages raises to 128VAC, the heater will increase from 6.3 to 6.72. ((128/120) x 6.3). Still within the +\-10% spec of the tube which is 5.67 to 6.93. The 6.93 upper limit is at 132VAC. the tube won't self destruct at higher heater voltages, but it does affect tube life.



Invalid Link Removed

 

[beans-on-toast]

I know it but I think that was when they were supposedly constantly cranked up to 11 by The Stones. 6146 family of tubes are not real reliable, I have a transmitter that uses them and I use 6293's instead the can handle a lot more sustained power. Those will sub for the older 6146's though I don't know about the 6146B's, I know the 6146B's not really interchangeable with the 6146' if they were I''''d use them as subs for the 6146B's in an SVT.s and 6146A's at least in ham transmitters.

I'd go with a 6146W over the 6146B that was initially used in the amp.

You must have dealt with the neutralization procedure required for the power tubes in transmitters. At high frequencies these tubes are prone to oscillation if they are not set up properly. Some people feel that 6146's are too much trouble to deal with. Not so much a concern in an SVT.

 

[cerrem]

I'd go with a 6146W over the 6146B that was initially used in the amp.

You must have dealt with the neutralization procedure required for the power tubes in transmitters. At high frequencies these tubes are prone to oscillation if they are not set up properly. Some people feel that 6146's are too much trouble to deal with. Not so much a concern in an SVT.

Exactly....the parasitic capacitance becomes an issue in the RF transmitters ....audio band with 47K input grid stoppers roll-off anything above 60kHz, so not a concern...
The 6146 tubes, when used within thier designed rattings are perfectly fine and reliable, whether it is in RF transmitters or Audio amps..
The original SVT driver circuit with 6146 tubes was not properly designed and the driver tube would saturate the 6146 grid with voltage greater then it's screen voltage on transients..Bang !!!
Once the SVT is re-wired so the driver tube plate is operated at the same rail voltage as the 6146 Screen voltage then the issue with blowing up goes away.... Never had any 6146 tubes blow in any of my personal SVT amps or any of the ones I have modified over the years... Zeners can also be installed and used as secondary protection to clamp transient voltages from preventing arcing in the 6146 tubes...
I have used 6146, 6146A, 6146W and 6146B in SVT amps with no problems....
Only difference is that the 6146B is 27 Watts of plate dissipation and all the other 6146 tubes are 20W Plate dissipation....
However, the SVT runs each output tube at roughly 15 Watts of plate dissipation.....
Some of the 6146W tubes are actually 6146B tubes screen for MIL usage...
I have used thermal imaging guns on the various 6146 versions and bulb temperatures well well in normal range below 220 C , except for 6146B which speced to go up to 260 C ...

 

[beans-on-toast]

The infrared thermometer can be used to roughly tell you how the plate dissipation compares for each tube. If some tubes are working harder than others. A well matched set will be close in temperature.

If you have a power amp that's acting up and one tube is way cooler than the others, then look there for a problem. It provides a rough way of evaluating what is going on before measuring the cathode currents.

 

[agedhorse]

In my 60's.tube amps, designed to run on 117V but frequently at 125V, I have almost 7 Volts on the heater string. As this is a reality of my local utility, I have installed resistors and diodes in my vintage amps to drop the heater voltage back into range. Those having a similar situation might be able to afford the cost and weight of a well regulated UPS, or are using commie tubes that are easily and cheaply replacable, but I use only US and western euro vintage tubes that are too expensive to sacrifice. YMMV.

This means that the heater supply was NOT properly designed, because if the heater voltage was in fact 6.3V at 117V AC input, the heater voltage will be exactly 6.74 volts, totally acceptable per the tube manufacturers. UPS tolerances are not generally all that tight as well, certainly worth verifying before using one if this is a big deal to you.

 

[JMacBass65]

You guys are just making stuff up now to see if anyone notices.

 

[JMacBass65]

It's like listening to my son talk about Pokemon.

 

[bobyoung53]

I'd go with a 6146W over the 6146B that was initially used in the amp.

You must have dealt with the neutralization procedure required for the power tubes in transmitters. At high frequencies these tubes are prone to oscillation if they are not set up properly. Some people feel that 6146's are too much trouble to deal with. Not so much a concern in an SVT.

The 6146B's won't neutralize a lot of the older ham transmitters

I'd go with a 6146W over the 6146B that was initially used in the amp.

You must have dealt with the neutralization procedure required for the power tubes in transmitters. At high frequencies these tubes are prone to oscillation if they are not set up properly. Some people feel that 6146's are too much trouble to deal with. Not so much a concern in an SVT.

No I never have, I should though because my transmitter (DX-100) oscillates up on the higher bands, 15M and above I believe, works fine where i used it though, 80 and 160M mostly.

 

[Korladis]

I'm not sure I understand all of this, but it's very interesting nonetheless.

 

[beans-on-toast]

The 6146B's won't neutralize a lot of the older ham transmitters

No I never have, I should though because my transmitter (DX-100) oscillates up on the higher bands, 15M and above I believe, works fine where i used it though, 80 and 160M mostly.

Yes the 6146B's can be a problem in rigs that weren't designed specifically for that tube revision. I recall a Collins unit that was particularly difficult to keep properly neutralized. Other transmitters, no problem at all.

Nice that you have a DX100 in service. I love old tube based radio equipment. I keep a R-390A reciever in my work area. What a work of art. A lot less shortwave stations are on the air these days.

 

[Korladis]

I assume that the 6146B is no longer made?

 

[beans-on-toast]

I assume that the 6146B is no longer made?

There is a Shuguang 6146B in production that cosst about $27-$32 depending on who is selling it. NOS US made ones from more than one manufacturer are available. Not sure if there are Russian equivalents. Sources that I've bought from are RF Parts and Invalid Link Removed. Other resellers stock them. They are one of the most popular transmitting tubes. Prices are reasonable because the hi-fi community doesn't use them.

These tubes cost less than 6550's. Some people debate if they should convert their older SVT's from 6146B's to 6550's. There seems to be two camps, one would only convert if 6146's were in short supply, the other very much prefers the 6550. Nice to have the choice.

 

[Korladis]

There is a Shuguang 6146B in production that cosst about $27-$32 depending on who is selling it. NOS US made ones from more than one manufacturer are available. Not sure if there are Russian equivalents. Sources that I've bought from are RF Parts and Invalid Link Removed. Other resellers stock them. They are one of the most popular transmitting tubes. Prices are reasonable because the hi-fi community doesn't use them.

These tubes cost less than 6550's. Some people debate if they should convert their older SVT's from 6146B's to 6550's. There seems to be two camps, one would only convert if 6146's were in short supply, the other very much prefers the 6550. Nice to have the choice.

Interesting. ~$30 a pop isn't bad. I don't have an SVT, so this is mostly an academic curiosity for me.

I imagine the tonal differences are minor?

 

[ThisBass]

As there is obviously some discord about the procedure how to measure MI amplifier output power,
I just like to refer to service manuals of a manufacturer with pretty high reputation.
The power amp test for a 200 watt MI bass amplifier reads as follows:

- Connect oscillator to input. ( 200Hz @ -46 dBV).
- Turn on 4 ohm load.
- Adjust volume for slight clipping( 2-3 o’clock). Output = 28 Vrms.

In another service manual for a 500 watt MI bass amplifier of the same manufacturer the power amp test reads as

- Raise the main VOLUME for slight clipping on the output, approximately 45Vrms


28Vrms results in 196 watt at 4 Ohm load for the 200 watt rated amplifier
45Vrms results in 506 watt at 4 Ohm load for the 500 watt rated amplifier


I wouldn't expect lots of difference if the output power is measured at 100Hz versus 200Hz but both methodes do not apply to the (more common) industry standard of 1kHz either way.

The industry standard also refers to output power at 1% THD.
The service manual does not refer to any THD. The only information given that ever applies to THD reads as "slight clipping".

IME there is a noticeable difference for the output power if the voltage is raised already to "slight clipping" versus "just before clipping". The measurable difference easily exceeds (or eats up) any difference caused by normal AC line voltage variation.

As there is no THD numbers provided within the service manual means it's the technician's responsibility about the way how to evaluate proper performance of the amplifier at 1/3 or 2/3 of rated output power.
Don't get me wrong, strightly speaking I'd claim that rated output power mentioned above as slightly glossed over due to the "slight clipping" method. At the other hand side these amps are well known for an extraordinary powerfull performance which leads to assuptions the rated specs were published in a more conservative way and "very relieable".


The situation is totally different if it comes to evaluate (respectively bench mark) pro audio amplifiers.
The manufacturers provide most often output power specs for 1%THD and 0.1%THD as well so it's quite easy to test the amplifier for it's rated performance and specs.
But while pro audio amplifiers are supposed to amplify with least of all THD possible to the contrary MI amplifiers are often designed to generate a distinct "coloration" and "feel" at high level volumes.
Pro audio amplifiers are supposed to amplify the program material with headroom even at high level volumes which leads to overpowering the cabinetry up to factor 4 in practice while MI amplifiers are often pushed very hard in practice which leads to considerably numbers of duty cycle (so there is no more headroom).

edit (mind thinking)
just to avoid THD consderations which IMO is (way often) rather counter-productive to evaluate output power of MI amplifiers. To compare apples to apples respectively pro audio amplifiers versus MI amplifiers it was possible to measure a 6db peak limited pink noise just before the peaks of the noise hit the rail voltage respectively can't get gained any higher.
I'm pretty sure amplifiers like the RH450 would perform way more different than any sine bench mark number would give reason to expect.

 

[beans-on-toast]

Interesting. ~$30 a pop isn't bad. I don't have an SVT, so this is mostly an academic curiosity for me.

The price is attractive which makes it easier to stockpile them.

Having a reliable and cost effective supply of tubes is important. It's a lot easier to find commonly used tubes such as a 6550, 6L6GC, 12AX7, etc. in a music store than having to order a 6146B (or W). Manufacturers today need a steady supply of tubes so they have adopted a very narrow range of tube types based on the best of what is available For instance, Loud found it better to go with 12AX7's rather than 6SL7's for the current reissue of the B-15. The 12AX7 is available from many suppliers, the sound quality is excellent. It wins over the 6SL7 when a manufacturer needs a large supply and low cost. The more they buy the better the price so it pays to use the same tube in multiple products. Same was true for the SVT amp family. Gone are the 12DW7 and 6C3 tubes, they have been replaced with more standard alternatives.

I imagine the tonal differences are minor?

Many claim that they can't tell the difference. But this can be a can of worms. They are different tubes and perform differently. Differences would be more evident when the amp is pushed. They are both very good, but when you are chasing tone, everyone has their favorites.

 

[beans-on-toast]

As there is obviously some discord about the procedure how to measure MI amplifier output power,
I just like to refer to service manuals of a manufacturer with pretty high reputation.
The power amp test for a 200 watt MI bass amplifier reads as follows:

- Connect oscillator to input. ( 200Hz @ -46 dBV).
- Turn on 4 ohm load.
- Adjust volume for slight clipping( 2-3 o’clock). Output = 28 Vrms.

In another service manual for a 500 watt MI bass amplifier of the same manufacturer the power amp test reads as

- Raise the main VOLUME for slight clipping on the output, approximately 45Vrms


28Vrms results in 196 watt at 4 Ohm load for the 200 watt rated amplifier
45Vrms results in 506 watt at 4 Ohm load for the 500 watt rated amplifier


I wouldn't expect lots of difference if the output power is measured at 100Hz versus 200Hz but both methodes do not apply to the (more common) industry standard of 1kHz either way.

The industry standard also refers to output power at 1% THD.
The service manual does not refer to any THD. The only information given that ever applies to THD reads as "slight clipping".

IME there is a noticeable difference for the output power if the voltage is raised already to "slight clipping" versus "just before clipping". The measurable difference easily exceeds (or eats up) any difference caused by normal AC line voltage variation.

As there is no THD numbers provided within the service manual means it's the technician's responsibility about the way how to evaluate proper performance of the amplifier at 1/3 or 2/3 of rated output power.
Don't get me wrong, strightly speaking I'd claim that rated output power mentioned above as slightly glossed over due to the "slight clipping" method. At the other hand side these amps are well known for an extraordinary powerfull performance which leads to assuptions the rated specs were published in a more conservative way and "very relieable".


The situation is totally different if it comes to evaluate (respectively bench mark) pro audio amplifiers.
The manufacturers provide most often output power specs for 1%THD and 0.1%THD as well so it's quite easy to test the amplifier for it's rated performance and specs.
But while pro audio amplifiers are supposed to amplify with least of all THD possible to the contrary MI amplifiers are often designed to generate a distinct "coloration" and "feel" at high level volumes.

edit (mind thinking)
just to avoid THD consderations which IMO is (way often) rather counter-productive to evaluate output power of MI amplifiers. To compare apples to apples respectively pro audio amplifiers versus MI amplifiers it was possible to measure a 6db peak limited pink noise just before the peaks of the noise hit the rail voltage respectively can't get gained any higher.
I'm pretty sure amplifiers like the RH450 would perform way more different than any sine bench mark number would give reason to expect.

No question that manufacturers employ different standards in characterizing the performance of their products. Nothing wrong with that in business.

I find that most players care more about how an amp sounds cranked with a band than any technical specifications. They know what they like and often want consistency. In the studio, hum and hiss are a concern. Some facilities offer studio amps that had been optimized for recording but the band brings in whatever they want. In the end, whatever the ear judges best is what prevails.

A tech is tasked with making the amp sound good and perform as it should. This requires test equipment to gather the necessary information. One might measure the power output voltage (or wattage), use a spectrum analyzer, a distortion analyzer, and a scope across a dummy load at the output. You want a very clean sine wave at the input so that it doesn't introduce any distortion. This combination of test equipment helps by providing checks and balances when evaluating the performance of an amp. When you look at the THD at the maximum power output, you can determine what is acceptable for that amp based on the specs. For instance, an unmatched set of power tubes will have a higher THD. It's one check that they are well matched. The spectrum analyzer will show any harmonics that should not normally be present. That might be caused by a bad capacitor. You want an approach that highlights issues.

 

[ThisBass]

IME the most noticable difference in tone with different preamp tubes appears if an inserted tube does not apply to the schematic.
IIRC with the 70's svt this appears for triode-system 12AU7 of V1 and V2 if the original 12DW7 preamp tubes are swapped for 12AX7 without conversion of the schematic.
With schematic conversion the difference in tone of 12DW7 versus 12AX7 is minor and likely as smallish as it may happen for different 12AX7.

There are some folks out there who try to get best gain as possible with any tube amplifier for the input stage and so (for all one is worth) try anything that might hit the goal.
So it's no surprise that bias may be shifted nearly out of order and likely hits a coloration (and of course considerably THD) that was not ment "this way" by the design engineer.

 

[agedhorse]

One problem with using a very low distortion figure is that if the amp is designed to highlight and emphasize those harmonic characteristics, the raw distortion may very well exceed .1% or 1% or even 5% in normal, desirable operation even though to power amp is or is not directly responsible for the distortion numbers. You could be stopping the test at an unintendedly low distortion threshold even though the product was designed with this specific attribute as part of its voicing.