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Tube Guitar Amp - Running Output Tubes in Spec?
Tube Guitar Amp - Running Output Tubes in Spec?
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Old 6th February 2019, 08:31 PM   #21
PRR is offline PRR  United States
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Quote:
Originally Posted by Printer2 View Post
I thought the cathode boils off electrons at a constant rate. Seems I was wrong.....
The heat just loosens the electrons. Why do they leave? Plate or screen voltage sucks them. Why don't they all leave at once? In normal operation the G1 voltage holds them back. There is an electron cloud, a virtual cathode, between K and G1. The electrons still in the oxide see this negative cloud and have no urge to leave the cathode.

The idea of higher V and lower I is pervasive in old tubes and particularly radio power tubes. Cathode stuff is expensive. With some qualification, plate voltage can be quite high. For a given Power (V*I), hi-V lo-I is a cheaper tube.

Fender Deluxe AA763 seems to exceed the highest numbers I know for 6V6.
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Old 6th February 2019, 09:06 PM   #22
nauta is offline nauta  Canada
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Hi

6V6 was used as a vertical deflection amp in old TVs where it had to withstand 1,200V pulses, so it's a pretty tough tube. 470V in a Deluxe is no problem. Flyback peak + DC never exceeds 800V considering how much the supply sags under load.

Even though the latest manual says it is design-maximum values, the sheet refers to variants up to GTA but still lists the design-centre values for the original metal version of the tube. Shame on RCA. No wonder even experienced guys are confused?

PRR: luv your posts. I can't believe I know something you don't but I did learn it from someone else
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Old 6th February 2019, 09:33 PM   #23
PRR is offline PRR  United States
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The 1967 sheet I linked says 350V design max. The design center rating for all older 6V6 is 315V. (Suggesting that center allows 10% for field variations.)

The HIGH-voltage Fenders come in the late 1960s, when 99% of tubes were made in about 3 plants. Leo may have had a gentleman's understanding that all the tubes he was likely to get, from whatever factory (all the brands horse-traded) would be made of 400V stuff, and he was not concerned about users using cheap replacements from odd sources.

There is no compelling reason to re-rate a tube if materials change. There is commercial reason to NOT re-rate, if it encourages sales of a similar tube sold at higher price. The 6V6 numbers are based on the 6F6 which it tended to replace, re-stated later as design-max instead of design-center (putting more onus on the OEM).

"Inductive kick" is a non-issue for symmetrical waves. Time above B+ same as time below B+... it averages out. (H-sweep waveforms are different.)

The "300V rating on most small tubes seems to be pure inertia. Small tubes rarely need more; when more is needed, usually a power tube is what is wanted. McIntosh treated 12AX7 to over 400V and it does not seem to be a problem.

The 300V numbers also come from standard radio practice. You had 450V e-caps. A vacuum rectifier. And then usually a 100V field-coil for more filtering. Taking 455V peak from PT, 55V drop in rectifier, 100V drop in field, is 300V. The older RF/IF tubes were usually worked from a 250V rail, a nice drop from 270V-320V at the Power tube. It all adds up, for the day and time. There's no benefit in amplifying sub-Volt RF/IF signals with high B+; even 250V is generous but it did increase the dynamic plate impedance for more gain and selectivity in IF tanks. But good radios were also made to work on 90V.

Last edited by PRR; 6th February 2019 at 09:35 PM.
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Old 6th February 2019, 09:39 PM   #24
Printer2 is offline Printer2  Canada
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Originally Posted by PRR View Post
The heat just loosens the electrons. <snip>
Thanks, I never really thought the cathode end of things through before.
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Old 6th February 2019, 09:44 PM   #25
aut0m4tic is offline aut0m4tic  United States
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The testbed is definitely not an ideal setup. I dont have much tube experience, and noticed that a lot of the old organ and console amps ran the screen within a few tens of volts of the plate. Reading RDH4 I realized that I dont understand the effect of screen voltage in a proper sense, so I used the smps so I could control the screen independent of b+, instead of it simply being a fraction of it, hoping to see the effect it has. Same with the bias voltage, it can be held constant, regardless of what b+ does. (Again, I'm assuming that a rectified bias source will sag when b+ does) I'm a serious case of not realizing how little I know. Probably just enough to make something work, without actually understanding why it works.

Cory
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Old 7th February 2019, 12:30 AM   #26
Printer2 is offline Printer2  Canada
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The screen sort of isolated the plate from the grid and cathode. Rather than the plate pulling the electrons to it the screen sets up an electric field that pulls the electrons from the cathode. In early amps they did not thing people will grossly distort the signal and turn the volume down when it does. For guitar amplification we do keep it turned up causing the plate voltage to dip low. When the plate is at a lower voltage it does not collect all the electrons that whizzed by the screen and the screen collects them as it is at a higher potential. This causes the screen current to go up at a high rate as shown on the graphs. We try to limit this by putting resistors in series with the screens. The increased current causes a voltage drop across the resistors and the screen voltage is reduced some, hopefully attracting less electrons and reducing screen temperature. Just a quickie explanation, others here are better at it.
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Old 7th February 2019, 03:10 AM   #27
BJosephs is offline BJosephs  United States
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Originally Posted by aut0m4tic View Post
The testbed is definitely not an ideal setup.
Your test bed looks better than my last few prototypes! I just lay everything on a wooden board and solder point to point until I like it.

Screen voltage has more of an impact on a pentode’s operating point than b+. A lower screen reduces gain and output power. That’s why good data sheets have a few sets of curves for different screen voltages. In fact IIRC you can estimate your negative bias by dividing the screen voltage by mu. If you regulate the screens you have to regulate the bias (and vice versa) but b+ can bounce around a bit.
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Old 7th February 2019, 05:09 AM   #28
aut0m4tic is offline aut0m4tic  United States
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Now that I have the power section operational, I need to add in a preamp. The chassis has several 12a?7 and several 6au6 sockets, so there are endless possibilities there. For starters, I'll build the preamp and tone stack on a "standard" fender tmb design (suggestions would be very appreciated), and see how it sounds.
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Old 7th February 2019, 10:12 AM   #29
thoglette is offline thoglette  Australia
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Originally Posted by aut0m4tic View Post
(suggestions would be very appreciated)
Head over to RobRobinette's site and have a look at the mini/micro amps for starters.

Champs have their own charm at one end of the trad-amp spectrum (1 valve, two knobs); JCM800 at the other (3 valves, full TMB and high gain).

Supro have a different tone stack set up.

Be careful, it's a slipperly slope!
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Old 7th February 2019, 12:26 PM   #30
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by nauta View Post
6V6 was used as a vertical deflection amp in old TVs where it had to withstand 1,200V pulses...
Yes, during flyback, when being cut-off, with no plate current, and just for a few microseconds...
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