fledgling builder questions

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cbevans
Junior tube assistant
Posts: 1
(3/17/04 11:05 am)
 fledgling builder questions
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 I am building a EL34 based 50-watt, plexi style amp and I have a few questions for any who can help.

1. AC volts on the lamp and heaters is reading 6.9 volts, is this acceptable?

2. 2nd preamp tube heaters aren't as bright as 1st preamp tube and phase inverter.....any thoughts?

3. Bias concerns... Plate Voltage: 520 volts. I have researched various different methods and opinions on this. One says that the ideal setting for plate voltages over 370 is 55mA. According to others, this is to high. "Inside Tube Amps" calls for about 25mA for this plate voltage. Please set me straight.

4. Once in a while the plates of the power tubes will glow red hot and the 1 ohm cathode resistors will read in the 300mA range. I immediately hit the standby and everything returns to normal when I take it off standby.....whats the issue here?

5. Buzz in the preamp (goes away when master vol is down). Everything in the preamp is grounded through the 12A jacks, what else can I do?

Thanks in advance
Berry
 
GroundhogKen
Forum Moderator
Posts: 2325
(3/17/04 1:25 pm)
 Re: fledgling builder questions
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 1. AC volts on the lamp and heaters is reading 6.9 volts, is this acceptable?

Seems a little high. Is this with tubes installed?


2. 2nd preamp tube heaters aren't as bright as 1st preamp tube and phase inverter.....any thoughts?

Assuming no wiring errors, different tube brands look different--with some tubes it's hard to see the filaments at all.


3. Bias concerns... Plate Voltage: 520 volts. I have researched various different methods and opinions on this. One says that the ideal setting for plate voltages over 370 is 55mA. According to others, this is to high. "Inside Tube Amps" calls for about 25mA for this plate voltage. Please set me straight.

520 when the amp is in play mode with tubes installed? That's high. What power transformer are you using? Are you in the USA--what's your mains voltage?


4. Once in a while the plates of the power tubes will glow red hot and the 1 ohm cathode resistors will read in the 300mA range. I immediately hit the standby and everything returns to normal when I take it off standby.....whats the issue here?

Very bad. Make certain your negative bias circuit is wired correctly. You should have about -35 to -45 volts on pin 5 of the power at all times--even in standby.


5. Buzz in the preamp (goes away when master vol is down). Everything in the preamp is grounded through the 12A jacks, what else can I do?

Let's solve the other issues first.

Ken
 
6G6
Junior tube assistant
Posts: 134
(3/17/04 1:25 pm)
 Re: fledgling builder questions
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 1. The lamp won't mind. The heaters might, but not right away.
2. Are all the preamp tubes the same age? Have you checked solder joints to the heater pins?
3. As the plate voltage goes up, the current needs to come down. 25mA sounds reasonable at 520V.
4. Having the bias at 55mA might have something to do with the red glow. Otherwise I dunno, but that is definately an issue!
5. Wire dress?
 
cbevans
Junior tube assistant
Posts: 2
(3/17/04 2:10 pm)
 Re: fledgling builder questions
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 1. 6.9 volts is with all tubes in place. I get the same reading at all sockets.

3. The power tranny is Hammond's 274BX (375 - 0 - 375 @175 ma.). It was recommended to me by Dan Torres. The plate voltage reading is 520 V with tubes installed and in play mode. Yes, I am in the USA, standard mains.

4. I will make sure that pin 5 is set right (change the bias range resistor if needed) and see if there are any differences.

Berry

 
GroundhogKen
Forum Moderator
Posts: 2326
(3/17/04 2:22 pm)
 Re: fledgling builder questions
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 520v sounds about right for that transformer.


With a 120v wall voltage, I would have used:

272HX (300-0-300 200mA 5V@3 A ct 6.3V@6 A CT)
for a 50 watt plexi

or

272JX (300-0-300 250mA 5V@4 A ct 6.3V@8 A CT)
for a 100 watter.

Ken
 
cbevans
Junior tube assistant
Posts: 3
(3/18/04 9:42 am)
 Re: fledgling builder questions
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 The problem with the power tubes overheating appears to be in a bad standby switch. It was only allowing about 10 V through on the line that feeds half the rectifers and the bias circuit. I replaced the switch and all looks well, but I am keeping a close eye on it.
 
 
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