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Tweed Champ circuit has me mystified...

10/14/1997 7:39 AM
Whit	Tweed Champ circuit has me mystified... Hi Y'all, Just put together a classic tweed champ circuit from scratch. 5Y3 12AX7 6V6. Have a SPST switch between pin 8 of the 5Y3 and the first filter cap, as a standby. First powerup test last night. With standby switch off, I measure an expected 370 V at pin 8. Flipping standby on increases this reading about 100 V, and all published V readings thruout the circuit are about 80-100 V too high (from schematic in Pittman) The 6V6 starts "melting" in a couple of minutes, although the amp worked well during those few minutes before it "farted out" and I noticed the glowing 6V6. As an admitted 6 month novice, this seems backwards, as I would expect a slight drop in B+ as the load is added. I'd greatly appreciate any clues from the experts here... but if all else fails, at least I live in the same city as Kevin O'Conner... ... Whit

10/14/1997 7:39 AM

Whit

Tweed Champ circuit has me mystified...

Hi Y'all,

Just put together a classic tweed champ
circuit from scratch. 5Y3 12AX7 6V6.

Have a SPST switch between pin 8 of
the 5Y3 and the first filter cap,
as a standby. First powerup test last
night.

With standby switch off, I measure an
expected 370 V at pin 8. Flipping
standby on increases this reading
about 100 V, and all published V readings
thruout the circuit are about 80-100 V
too high (from schematic in Pittman)
The 6V6 starts "melting" in a couple of
minutes, although the amp worked
well during those few minutes before it
"farted out" and I noticed the glowing
6V6.

As an admitted 6 month novice, this seems
backwards, as I would expect a slight drop
in B+ as the load is added.

I'd greatly appreciate any clues from the
experts here... but if all else fails,
at least I live in the same city as
Kevin O'Conner...

... Whit

10/14/1997 8:46 AM
J Fletcher	Hi Whit...is there a filter cap in circuit when the standby is off?If there isn't,then it is normal for the dc voltage to rise as it charges up towards the peak voltage of the pulsating dc.You should have a dc voltage of approx 1.25 of your ac high voltage.ie a 300-0-300 winding will give you around 375 volts dc after a full wave rectifier into a cap input,with a load connected.....see you around...Jerry Fletcher...

10/14/1997 8:46 AM

J Fletcher

Hi Whit...is there a filter cap in circuit when the standby is off?If there isn't,then it is normal for the dc voltage to rise as it charges up towards the peak voltage of the pulsating dc.You should have a dc voltage of approx 1.25 of your ac high voltage.ie a 300-0-300 winding will give you around 375 volts dc after a full wave rectifier into a cap input,with a load connected.....see you around...Jerry Fletcher...

10/14/1997 9:23 AM
Whit	Jerry, Thanks for the response, you may have just clued me in. The standby is actually before the caps, but my trouble may lie elsewhere. The PT I'm using is one I (possibly mistakenly) chose from Angela Inst. web site. It was described as a Champ/Princeton PT. It's a 370-0-370 wired in a standard way to the 5Y3 (except I have 2 diodes inline with the RED wires for protection). So given the 1.25 multiplier, I get about 460 V of B+, which as I understand it is WAY too much for the plate of a 6V6. And is in line with what I measured. Could it be that this transformer is intended for later champ/princetons with SS rectifiers and I chose poorly? (I'm trying to remember all the SS vs tube rectification theory here, but nothing's happening!). Does this sound like a proper diagnosis? Always learn more from mistakes anyways! ... and thanks again... Whit

10/14/1997 9:23 AM

Whit

Jerry,

Thanks for the response, you may have
just clued me in.

The standby is actually before the
caps, but my trouble may lie elsewhere.

The PT I'm using is one I (possibly
mistakenly) chose from Angela Inst.
web site. It was described as a
Champ/Princeton PT.

It's a 370-0-370 wired in a standard way
to the 5Y3 (except I have 2 diodes inline
with the RED wires for protection). So given
the 1.25 multiplier, I get about 460 V of
B+, which as I understand it is WAY too
much for the plate of a 6V6. And is in
line with what I measured.

Could it be that this transformer is
intended for later champ/princetons with
SS rectifiers and I chose poorly?
(I'm trying to remember all the SS vs
tube rectification theory here, but
nothing's happening!).

Does this sound like a proper diagnosis?
Always learn more from mistakes anyways!

... and thanks again... Whit

10/14/1997 10:12 AM
Mike T	460 Volts is a lot for a 6V6, especially biased into Class 'A'. But not for a 6L6 or 5881, or if you do some minor rewiring of the socket, EL34. Mike

10/14/1997 8:54 PM
Bruce	Be carefull if you simply replace the 6V6 with an EL34.... the filament current is much higher when compared to a 6V6. This could put an unwanted addtional load (heat) on the PT and cause it to burnout prematurely. Just check the specs on the PT and look to see if the 6.3v sec winding can safely supply all the extra current that your asking it to. Also try using a 5U4 instead of a 5Y3. I think it has a larger voltage drop then a 5Y3. You could always try the hi wattage zener diode trick in the center tap of the PT to ground. That will drop the output voltage of the power supply down by a few 10's of volts with the right zener diode. A reasonable cost effective way to do that. ABout $10.00 Or R.G. Keen's idea of a voltage bucking transformmer on the primary to drop the secondary down 10vac-20vac. Class A self biased ?....... Measure the voltage on the plate of your 6V6 against the cathode/resistor junction instead of plate to ground. That is the real potential on the plate you're interested in. You might not be off as far as you think. What is the voltage of the cathode to ground? Subtract that from the delivered plate voltage when the tube fully warmed up and drawing bias current. Anyone else? Bruce

10/14/1997 8:54 PM

Bruce

Be carefull if you simply replace the 6V6 with an
EL34.... the filament current is much higher
when compared to a 6V6. This could put an
unwanted addtional load (heat) on the PT and
cause it to burnout prematurely.
Just check the specs on the PT and look to see
if the 6.3v sec winding can safely supply all
the extra current that your asking it to.
Also try using a 5U4 instead of a 5Y3. I think
it has a larger voltage drop then a 5Y3.
You could always try the hi wattage zener diode
trick in the center tap of the PT to ground.
That will drop the output voltage of the power
supply down by a few 10's of volts with the right
zener diode. A reasonable cost effective way to do
that. ABout $10.00
Or R.G. Keen's idea of a voltage bucking
transformmer on the primary to drop the secondary
down 10vac-20vac.

Class A self biased ?.......
Measure the voltage on the plate of your 6V6
against the cathode/resistor junction instead
of plate to ground.
That is the real potential on the plate you're
interested in.
You might not be off as far as you think.
What is the voltage of the cathode to ground?
Subtract that from the delivered plate voltage
when the tube fully warmed up and drawing bias
current.
Anyone else?
Bruce

10/14/1997 9:00 PM
Bruce	I just re-read your message and I wonder what value screen resistor you are using? I find these little homebrew amps can run at a higher voltage with a larger value cathode resistor and screen resistor of up to 1K5. Measure those voltages again and repost. I am running an SE Champ clne at 390V as measured from the plate to the cathode pin. It has been running like that for weeks. I turn it on in the AM at my shop and leave it on all day and goof around with it through out the day. Bruce

10/14/1997 9:00 PM

Bruce

I just re-read your message and I wonder what
value screen resistor you are using?
I find these little homebrew amps can run at a
higher voltage with a larger value cathode
resistor and screen resistor of up to 1K5.
Measure those voltages again and repost.
I am running an SE Champ clne at 390V as measured
from the plate to the cathode pin.
It has been running like that for weeks.
I turn it on in the AM at my shop and leave it on
all day and goof around with it through out the
day.
Bruce

10/15/1997 8:12 AM
Whit	Bruce, Thanks for the good info, I'm learning far more now than if the amp had worked perfectly the first time. The Tweed Champ schematics, both on Ampage and Pittman's book, are what I used to construct this amp, so those are the values in the circuit. I will measure all relevant voltages tonight and repost tomorrow. I intend to stay with the 6V6 instead of changing the output tube type. I am still working on the premise that the 375-0-375 tranny I employed is just too much, and hence throwing too much voltage at the entire circuit. I'm considering adding a PI filter, and/or another R/C pair at the front of the filter section . ie. decrease B+ before it enters the existing filter section if that makes sense. Will a choke decrease B+ ? Thanks again for the attention... ... Whit

10/15/1997 8:12 AM

Whit

Bruce,

Thanks for the good info, I'm learning
far more now than if the amp had worked
perfectly the first time.

The Tweed Champ schematics, both on
Ampage and Pittman's book, are what I
used to construct this amp, so those
are the values in the circuit.

I will measure all relevant voltages
tonight and repost tomorrow. I intend
to stay with the 6V6 instead of changing
the output tube type.

I am still working on the premise that
the 375-0-375 tranny I employed is just
too much, and hence throwing too much
voltage at the entire circuit.

I'm considering adding a PI filter,
and/or another R/C pair at the front of
the filter section . ie. decrease B+ before
it enters the existing filter section
if that makes sense.
Will a choke decrease B+ ?

Thanks again for the attention...

... Whit

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