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Old 19th November 2011, 08:52 PM   #1
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Default ideas for a tube eating guitair amp

well . here it goes . i'd like to build an amp that eats TV tubes for breakfast .so i was thinking what kills a tube in the fastest manner possible ?

it would be fun putting a lot of tv tubes on the edge at lowish voltages and high current maybye you could use a 20-20Vac transformer to get near 50 volts of plate supply also use it to supply the heater current

it seems that if you use a PL84 at 50 volts B+ whit zero grid bias . it will draw 300Ma in idle conditions something like five of these paralel should be able to supply current no output transformer ofcourse and some crappy speaker as a lamb to the slaughter

input stage could be anything pcf i have a TON of these not really useful for anything

anyone have any ideas to make something fun out of these box filling space wasting P valves ?

Last edited by v4lve lover; 19th November 2011 at 08:59 PM. Reason: edit i hit enter to fast
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Old 19th November 2011, 10:44 PM   #2
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Quote:
what kills a tube in the fastest manner possible
It's usually the floor!

Quote:
anyone have any ideas to make something fun out of these box filling space wasting P valves ?
IF you think all of the P valves are wasting your space, we can work out a trade since I have made a rather nice sounding 18 watt guitar amp with them that doesn't eat tubes.

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PL84 at 50 volts B+ whit zero grid bias
Try 320 volts on the plate and 160 on the screen with cathode bias using a 150 ohm resistor for both tubes and a 6600 P-P OPT.

The amp was designed for this thread:

The Hundred-Buck Amp Challenge

The preliminary schematic is in post# 794. It has been modified a lot since then/ I will post a new one when I have time.
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Old 19th November 2011, 11:17 PM   #3
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OK, the PL84 is a 300 mA. series string version of the EL84/6BQ5. Build an "El Cheapo" variant with a 300 mA. series heater string.

4X PL84s "eat" 60 V. A pair of 12AT7/ECC81s "eat" 12.6 V. at 300 mA. Finally, a 6AL5 eats 6.3 V. That leaves 41.1 V. to drop, assuming North American power mains. You could get "cute" and use a capacitance, instead of a resistance, as the dropping element.
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Old 20th November 2011, 08:21 AM   #4
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Wrong . the PL84 is a series variant of the EL86 a low resistance low voltage pentode two el86 used to drive some odd 800 ohm speakers

the only thing telling a series tube and a E tube apart is the more rugged as in Thicker heater in a series variant .

cathode insulation is also exeptionally good whit 250volts quoted in alot of datasheets

and if anyone need some i have used examples . i can test for cathode emission
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Old 20th November 2011, 01:16 PM   #5
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I should have looked at the data sheets closely. This is another case of inconsistency, like 6L6/50L6.

OK, no "El Cheapo". However, the PP AB1 setup and 300 mA. series heater string ideas remain valid. Bridge rectifying a Triad N-68X isolation trafo will take care of B+, quite well. Scott/Dyna style circuitry should do. How about a 6AU6 voltage amplifier DC coupled to a ZVN0545A "concertina" phase splitter, for the small signal stuff. 0B2 regulating PL84 g2 B+ would improve open loop linearity.
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Old 20th November 2011, 02:51 PM   #6
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Originally Posted by tubelab.com View Post
It's usually the floor!

IF you think all of the P valves are wasting your space, we can work out a trade since I have made a rather nice sounding 18 watt guitar amp with them that doesn't eat tubes.

Try 320 volts on the plate and 160 on the screen with cathode bias using a 150 ohm resistor for both tubes and a 6600 P-P OPT.

The amp was designed for this thread:

The Hundred-Buck Amp Challenge

The preliminary schematic is in post# 794. It has been modified a lot since then/ I will post a new one when I have time.
I picked up a few to use in a SE amp that have 1100 ohm primaries which I was going to double by using 16 ohm speakers. That might change when I see your circuit. 320V on the plate? And I thought Fender had a thing for going over voltage. Thinking about it, does the amp sound similar to a EL84 stuffed amp? Have you tried it at lower voltages in p-p with lower primary impedance?
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Old 20th November 2011, 04:09 PM   #7
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I messed up since I am not used to the European tube numbering system. My amp does not use P valves, it uses U valves. Either way the output stage is the same. First off the numbers:

EL84 = 6BQ5

XL84 = 8BQ5 which is an 8 volt 6BQ5

I have seen 10BQ5's but do not know the european number. They are not common. I have never seen a 15BQ5.

EL86 = 6CW5

XL86 = 8CW5 which is an 8 volt 6CW5

LL86 = 10CW5 which is a 10 volt 6CW5

PL84 = 15CW5 which is a 15 volt 6CW5. It is NOT an EL84 variant.

UL84 = 45B5 which is a 45 volt 6CW5. It is NOT an EL84 variant.

A 6CW5 looks like an EL84 and does indeed have the same plate structure. The screen grids are different, with very different ratings. Plugging a 6CW5 into an EL84 socket will result in tube meltdown with possible other fried parts.

OK, so we all know how to make EL84's and their variants work. Many amplifiers did indeed violate the plate voltage on EL84's often by a lot. I have seen ads for a HiFi amp that claims to run them at 700 volts! The trick to avoiding nuclear meltdown with these tubes is to keep the screen voltage under 320 volts, and even lower if you abuse the plate.

Well the 6CW5 family has different ratings, but responds the same way.

Quote:
Have you tried it at lower voltages in p-p with lower primary impedance?
Yes, I have building my SPP HiFi amps with 6CW5's and I run about 235 volts on plate and screen with a 3300 ohm load for about 15 watts of good clean power. With the guitar amp experiments in the HBAC thread I found that it sounds more like a screaming EL84 if you feed it like one. THis means more volts and more ohms. I use 320 volts on the plate because that is easy to get with a voltage doubler on an isolation transformer. To avoid meltdown at that plate voltage the screen must be lower, again 160 volts is easy to get, so that's what I used. More power can be had with the plate around 400 volts and the screen at 125 or so. I haven't blown one up yet so I don't know where the limit is.

The output stage and PI hasn't change since I posted that schematic. I am just tweaking the uber gain stages in the preamp. Again U tubes UCC85's.
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Old 20th November 2011, 04:17 PM   #8
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PL84 = an el86 only thing to tell them apart is thicker coiled heater wire on the 84

they can do low voltages as intended internal resistance is also pretty low

HMM PD510 also looks nucleair even has WARNING X rays on the side ..
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Old 20th November 2011, 04:22 PM   #9
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here's a nice site whit datasheets on european tubes John's Radio Web edit might want to put it through google translate exelent info on european tubes in the P and E directory if you need help acquiring them i might be of assistance whole BOXES full of tv tubes show up localy and cheap

edit anyone want to experiment whit PCF80 802 ? 82 triode/ penthodes wich are available for almost nothing

Last edited by v4lve lover; 20th November 2011 at 04:36 PM.
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Old 20th November 2011, 05:13 PM   #10
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HMM pcf80's will take 30 volt heating wich is 22 over spec for some time .. nice stuff
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