Hendrix's tubes

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Re: Hendrix's tubes

Post by Roe » Wed Oct 03, 2012 12:46 pm

marshall switched to 6550s around 1974 on the american amps
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Re: Hendrix's tubes

Post by frenchie » Thu Oct 04, 2012 7:03 am

Xplorer wrote:hi, i didn't notice it before, but now i wonder about the tubes i see in the 7026. they don't look like kt66. what do you think they are please ?
they are 7581A tubes ... aka reinforced 6L6s

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Re: Hendrix's tubes

Post by Xplorer » Thu Oct 04, 2012 7:23 am

thanks ! so this could be what hendrix used in the usa maybe .. ( unless rich changed these tubes )
KT66 are some kind of version of the 6l6 , right ?

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Re: Hendrix's tubes

Post by frenchie » Thu Oct 04, 2012 7:30 am

Xplorer wrote: KT66 are some kind of version of the 6l6 , right ?
yep but KT66s and 7581As are able to manage a lot more current , hence their presence in 560V B+ amps like the 45/100 , an amp in which the standard 6L6s would fry , or live really short lives for the best of them ... KT66 was the english version of 6L6s ( created it probably in order not to have to import them ) , 7581A was an american military strenghtened version of their 6L6s ...

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Re: Hendrix's tubes

Post by Xplorer » Thu Oct 04, 2012 7:36 am

oh ! so i should get a quad of these 7581A, it might be cheaper than the gec kt 66!?

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Re: Hendrix's tubes

Post by Roe » Thu Oct 04, 2012 12:09 pm

actually, the date sheets for the 7581As suggests that it cant handle the voltages of the 45/100s. max screen voltage is 450v. and the 559v of the 45/100 would be risky.
the 7581As have a cleanish, harder tone that isn't for everyone (I happen to like them and have bought two NOS quads)
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Re: Hendrix's tubes

Post by Xplorer » Thu Oct 04, 2012 12:22 pm

What NOS power tubes would you suggest as an alternative for the gec kt66 at high voltage ?

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Re: Hendrix's tubes

Post by Roe » Thu Oct 04, 2012 12:26 pm

Xplorer wrote:What NOS power tubes would you suggest as an alternative for the gec kt66 at high voltage ?
you don't need NOS but the only safe solution seems to be kt88s. however, if you can bring down the screens voltage, then you can use 6l6s, 6550s, el34s. the new gold lion kt66s changed specs. max screens voltage is now 400v, max plate is 500v. I run mine at 530 and 535 respectively
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Re: Hendrix's tubes

Post by Xplorer » Thu Oct 04, 2012 12:41 pm

Thank you

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Re: Hendrix's tubes

Post by Roe » Fri Oct 05, 2012 3:42 am

keep in mind however that the voltages are only real high at idle. these amps sag a lot when you turn up the volume. the screens sag a lot with the RS 690 ohms choke, mine drop from 530v to ca 400v at pin 4 of kt66s
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Re: Hendrix's tubes

Post by Tone seaker » Sat Oct 06, 2012 11:12 am

I run my KT66 gold lions in my super JH 100 at 525V with no issues

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Re: Hendrix's tubes

Post by daveweyer » Mon Nov 03, 2014 10:48 pm

Probably a dead thread but this discussion could stand some corrections. There is an oft repeated quote from an interview I did with Art Thompson at Guitar Player Magazine regarding the services of West Coast Organ and Amplifier Service on Jimi Hendrix' equipment, the same quote appearing on a number of different forums and containing some factual, and probable accidental errors concerning speakers we installed in Hendrix' Marshall cabinets and tubes we used in his Marshall amps among other modifications.
The Rola speakers which I obtained from Thomas Organ Company and installed in the cabinets were not 75 watt rated, and I am not aware of a Rola speaker which had such a power rating. There were Altec speakers with 75 watt ratings but Thomas did not like to use them because they cost too much. I did not like to use them because in guitar amps they were over-damped, just like the JBL speakers. And Jimi did not like them either, which was a lesson learned by Sunn when they furnished his equipment for a short while. Highly damped speakers with large magnets and large voice coils took all the "liveness" out of the guitar/amp interaction, and didn't have that special "tone" which many guitar players love.
Thomas had a huge stock of Rola speakers, and one model, which engineering claimed had a higher power rating than the normal T 1088, but looked identical and was ostensibly used in bass heavy applications, (and had no Vox sticker over the screw) were used in our bulk replacement services. Bob Hovland, who was an engineer at Vox under Brad Plunket tested those speakers with me at the lab at Thomas Organ Company one evening in 1969. We were able to pump 60 peak watts through them, although they probably had considerably less RMS power rated voice coils. These speakers had impedance specifications allowing them to be paralleled in groups of four, which was very handy for matching high powered amplifier output impedances, and they also had a bit darker tone, which Jimi preferred, in both his speakers, and his Wah Wah pedals.
So what did Jimi gain by the replacement of the stock Marshall Celestions with these speakers? Probably not much except for the change in tone, although we knew that these speakers in groups of four would take 120 watts all night long, and that made us feel better about sending them out. In fact Bob and I had hooked up ten 120 watt amps to forty of these speakers one night at Thomas Organ Company just to see what we could shake loose from the rafters at full output, we got quite a lot I might add, a literal dust storm.
When the shipment of Marshalls arrived at West Coast Organ and Amplifier Service in 1969, after Marshall had agreed to provide Jimi with equipment, there were several other Marshall heads and cabinets which came in at the same time. These were literally broken to pieces, and were all rebuilt and recovered with Marshall fabric to match the new ones, and the original Celestions were replaced with all the spares we had left over. So no one really knows which cabinets were used with which heads, suffice to say there was a mixture of different models and parts, and I'm not sure if all the new cabinets got the Thomas Rola speakers.
There were guitars, Wah Wah pedals and Fuzzface units in crates of 20, and lots of other road gear to repair or modify.
Neal Moser and Rick Mierop helped me get the whole touring package ready for the Experience by the way.

Now the 6550 story. This is a bit arcane, but the two major factors in using 6550s were reliability and loudness. The third factor was tone. Jimi had been trying to find a way to get around that recurring situation of being in the middle of a solo or jam, being totally lost in his music, and have his amp die or blow up on stage. He said it totally disrupted his energy and brought him down from whatever light beam he was on, and ruined the groove he was in. There are several interviews where he talks about this.
He asked me if I could help remedy the situation. Now the arcane part. In Marshall amps, and other copies of old Fender style designs, the plate and screen grid are operated at approximately the same voltage. This makes the tube very sensitive, less drive voltage required, but puts the screen grid in a situation of drawing tremendous current for the part of the cycle that the plate voltage is lower than the screen, which is most of the cycle in amps which run the plate and screen at the same voltage. Since guitar service in loud rock situations requires the amp to live in essentially square wave clipping for much of its stage time, the screen grids over-dissipate, get red hot, sag and touch another element of the tube, blowing something out, or even melting the plate. When a 100 watt tube amp is run into full clipping, it can easily put out 150 watts, so a reliable design has to consider this.
The Guitar Player interview mistakenly used the term "suppressor" when it should have said "screen" grid, by the way. 6550s only have a beam plate in place of the suppressor grid found on an EL34.
So for one issue, EL34 screen grids would get too hot and disintegrate because the plate couldn't get rid of the heat, the tube rated a good ten watts less dissipation than a 6550.
The 6550 also had more current drawing capability, a larger cathode, and could deliver more absolute power.
The way to truly solve the problem and deliver even more output power is to lower the screen voltage and raise the plate voltage, which is what I did to a number of Jimi's amps, (and a lot of other amps) a point not well explained in the interview and a point seemingly ignored in the guitar amp community. This works on any power tube but requires a separate supply or regulator; the only company apparently willing to do this in the guitar world was Guild, which used a tube regulator for the screen grids of 8417 output tubes used in its Quantum amps, which Jimi used by the way in his earliest days with the Experience, and which West Coast Organ and Amplifier Service repaired and modified in 1968. (Some of this Guild equipment was given to Cat Mother later in Jimi's career, and both the J.H. EXP initials and Cat Mother can be seen stenciled onto the equipment)

The tone. As Jimi's career progressed, he began to like a harder edged tone because it worked so well for blues, quite clean until a certain threshold was reached, then a very rapid ascent into hard distortion. The 6550 has a resistance to breakup until a certain point is reached, then some quite harmonically rich distortion occurs. Aspen Pittman has talked rather extensively about this in his observations about tubes and their tone.
For a smooth gradually increasing distortion, the EL34 (and its cousins) works very well.

No one could ever know exactly what tubes, speakers, and effects were used on any particular Hendrix performance, because there were techs all over the country who could have pulled out everything I put in Jimi's amps and replaced some or all with something they liked or thought was better. But I do know that Jimi thought the amps with 6550s were more reliable, and they probably were.

The story is much bigger and more complex of course, but these are basic facts of the often discussed questions regarding Jimi's speakers and tubes from West Coast Organ and Amp Service.

David Weyer, Chief Tech at West Coast Organ and Amplifier Service

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Re: Hendrix's tubes

Post by Xplorer » Tue Nov 04, 2014 3:54 am

Wow ! That's first hand infos !!
Thanks so much David !

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Re: Hendrix's tubes

Post by Roe » Tue Nov 04, 2014 5:44 am

daveweyer wrote:...this discussion could stand some corrections...Thomas had a huge stock of Rola speakers, and one model, which engineering claimed had a higher power rating than the normal T 1088, but looked identical and was ostensibly used in bass heavy applications...
The way to truly solve the problem and deliver even more output power is to lower the screen voltage and raise the plate voltage, which is what I did to a number of Jimi's amps...

David Weyer, Chief Tech at West Coast Organ and Amplifier Service
Thanks for sharing, David!

Could you elaborate a little on the speakers and on the power supply mods? Were you using the 30W G12H 55HZ or the 25W G12M 55HZ speaker (or something else) in Jimi's guitar cabs? And how did you lower the screens voltage while raising the plate voltage? Did you install a new power transformer with dual secondaries? (if so, any idea about the specs?)

thanks!

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Re: Hendrix's tubes

Post by daveweyer » Tue Nov 04, 2014 11:57 am

Plates and screens. There are two basic ways to deal with over-dissipation of screens, and one very simple idea is to limit the screen current by using a 2K 10 watt resistor on each screen, something I did on several of Jimi Hendrix' amps.
When the amp is in hard clipping mode, the screen current can rise by a factor of ten because the screen is trying to do the work of the plate; the 2K resistors cause a substantial drop in the screen voltage whenever the instantaneous plate voltage is below the screen voltage. This limits total output power of the circuit, but it saves the screens, and the output tubes, and also gives a very nice sort of compression, like power supply sag (which Neil Young loved and called "sucking").
The second way to control screen current is to use separate supplies, or a regulator similar to what Guild used on their Quantum amps with 8417 output tubes; their circuit used a 6GF7 series regulator which essentially provided 300 volts to the screens at any output operating level, and allowed them to use 560 volts on the plates of the output tubes without having to worry about over-dissipation of the screens. They could actually get 100 watts from this design using only two tubes, and count on good longevity.
When I received Jimi Hendrix' Guild Quantum amp for repair in 1968, the transformer was blown, but the output tubes were still okay; I was surprised and remembered the circuit as being special.
You can also use shunt regulation to keep the screen voltage at a predetermined level, the system wastes heat, but is extremely fast and quite simple to implement. Essentially, the circuit uses a device, like a large Zener, or zener/transistor combo to substitute for the the maximum current draw of the screens, meaning there is substantial heat generated when the amp is at idle. It's really a dummy load that draws current through a dropping resistor, when the screens begin to draw current the dummy load drops out of the picture gradually until the screens are drawing all the current in the dropping resistor. This holds the screens at a constant voltage within the operating range.
I installed one of these circuits in one of Jimi's Marshalls, and I think it worked out pretty well. Somebody somewhere has this Marshall if it didn't get trashed and probably wonders what the strange circuit is all about.

In the 1950s the "ultra-linear" circuits became very popular in the Hi-Fi world, and the KT and EL series of output tubes were targeted for these circuits, one notable feature of which was the plate and screen working at the same voltage, both connected to taps on the output transformer. When you check the plate voltage on these amp designs you will see it maintained in the more comfortable 400 volt range, precisely for longevity of the output tubes.
These amps were never intended to operate in hard clipping either, which is something the designers of guitar amps didn't think that much about when they basically copied these designs and simply raised the plate voltages and currents to get the desired output power.

I'll deal with specific mods and the storied "Governor" amp which originated at West Coast Organ and Amp Service, in a future post.
DW

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