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From cign–(at)–elios.phy.OhioU.Edu Thu May 16 13:24:00 CDT 1996
Article: 9803 of rec.audio.tubes
Newsgroups: alt.guitar.amps,rec.audio.tubes
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From: cign–(at)–elios.phy.OhioU.Edu (Dave Cigna)
Subject: zener diode for dropping B+
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Against many recommendations I bought a 56V, 50W zener and installed it
in my Princeton Reverb. This is a stud mount device with DO-5 case that
requires a 5/16 inch hole. I bolted it directly to the chassis without
any heat sink. The HT center tap was lifted from ground and attached to
the terminal of the zener.

With a GZ34 rectifier, B+ at the first filter cap dropped from 453V to
404V. The voltage on the plate of the first preamp tube dropped from
177V to 158V. There is a DC drop of 43V across the diode plus about
20VAC. The AC is due to the fact that the rectifier – and hence the
transformer – is not conducting during the entire AC cycle. I bypassed
the diode with a .1uF cap because it makes me feel better. If anyone
can justify this, or explain why it shouldn’t be there, I am happy to
listen.

I installed a toggle switch to short the diode so that I could A/B
back and forth. I could find no additional ripple or hash on the B+
with the zener in circuit. The cap didn’t change anything on the B+
side of things, but it did shunt the AC at the center tap.

This amp is cathode biased (a previous attempt to bring plate-to-
cathode voltage down), so this modification didn’t affect the bias
in any way. Amps that use a separate tap for fixed bias might need
to have the bias supply reworked using one of the HT taps. I think
that amps that are already using a HT tap for the bias supply will
not require any changes.

The diode gets warm, but it’s not uncomfortable to touch continuously.
In this installation it is passing only about 50mA. It would certainly
get hotter in a Twin Reverb, but I don’t think that a heat sink would
be required if it is mounted to an uncluttered part of the chassis.
Perhaps the back panel would be a good place.

How does the amp sound? Great! Finally, I like to play this amp the
way that I always felt that I should. The hard edge is gone and the
springyness that I love about 6V6s comes through. This confirms what
I have noticed before: 6V6 tubes just don’t sound their best with
high plate voltages. They start to lose their 6V6-eyness with much
more than about 400 volts. About 375V plate-to-cathode sounds best to
my ears. In this case, the zener plus the cathode bias put it right
on the money.

Despite all the previous warnings against using a zener to drop B+,
I am very happy with the results. Except for one small hole in the
chassis, the mod is easily reversible. I may convert a seldom used
Showman to 6V6s …

— Dave Cigna

From mgarvi–(at)–anix.com Thu May 16 13:24:10 CDT 1996
Article: 9820 of rec.audio.tubes
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps,rec.audio.tubes
Subject: Re: zener diode for dropping B+
Date: 16 May 1996 12:07:51 -0400
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In cign–(at)–elios.phy.OhioU.Edu (Dave Cigna) writes:
>Against many recommendations I bought a 56V, 50W zener and installed it
>in my Princeton Reverb. This is a stud mount device with DO-5 case that
>requires a 5/16 inch hole. I bolted it directly to the chassis without
>any heat sink. The HT center tap was lifted from ground and attached to
>the terminal of the zener.

Hi Dave,

The chassis should make an adequate heat sink. If you were
referring to my comments about derating for open-air operation, I
meant that literally…no heat-sinking. The chassis will conduct
quite a bit of heat away from the zener.

>This amp is cathode biased (a previous attempt to bring plate-to-
>cathode voltage down), so this modification didn’t affect the bias
>in any way.

That’s a key point that’s overlooked sometimes. Anything done to
to center tap of the secondary will affect a bias tap on that same
secondary.

>The diode gets warm, but it’s not uncomfortable to touch continuously.

>Despite all the previous warnings against using a zener to drop B+,
>I am very happy with the results. Except for one small hole in the
>chassis, the mod is easily reversible. I may convert a seldom used
>Showman to 6V6s …

Hmmmm…I hope you weren’t referring to my comments. I use ’em
sometimes. I remember posting warnings about some aspects, such
as the bias thing and dangers of floating a large heat sink at
400v (for series operation). Hope it didn’t sound that ominous.

There are people who warn about noise generated by the reversed
junction. I have never seen a problem, except possibly in some
esoteric preamp configurations (voltage translators in cascodes).
Any low-level zener noise should be filtered easily in your
configuration.

M.Garvin

From SRSN81–(at)–rodigy.com Thu May 16 15:34:14 CDT 1996
Article: 15209 of alt.guitar.amps
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From: SRSN81–(at)–rodigy.com (Joseph Pampel)
Newsgroups: alt.guitar.amps
Subject: Re: zener diode for dropping B+
Date: 16 May 1996 16:59:02 GMT
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cign–(at)–elios.phy.OhioU.Edu (Dave Cigna) wrote:
>
>Against many recommendations I bought a 56V, 50W zener

Just out of curiosity, who was against it? I use them all the time. They
work great.

> I bolted it directly to the chassis without any heat sink.

some would say the chassis :is: the heat sink. 🙂

>There is a DC drop of 43V across the diode plus about
>20VAC. The AC is due to the fact that the rectifier – and hence the
>transformer – is not conducting during the entire AC cycle. I bypassed
>the diode with a .1uF cap because it makes me feel better. If anyone
>can justify this, or explain why it shouldn’t be there, I am happy to
>listen.

There should be a .1uF film cap there just to make sure that you don’t
get hash from the zener firing noise. It should be mounted right at the
zener. (as close as possible anyhow) I think Dr. Distortion told me once
that he likes to bypass with an even bigger cap (10uF?) just to tie the
PT CT to AC ground better. FWIW, I’ve never (knock wood!) had trouble
with a zenered power supply.

>Despite all the previous warnings against using a zener to drop B+,
>I am very happy with the results. Except for one small hole in the
>chassis, the mod is easily reversible. I may convert a seldom used
>Showman to 6V6s …

I would just add that Mojo sells a Tweed Twin PT that not only has a
rectifier tap (5AR4 anyone?) but it also has 2 secondarys, a HV one for
6L6’s, and a lower voltage one suitable for 6V6’s.. Imagine a 2×12 combo
with 4 6V6’s running cathode bias.. a Super Deluxe. ==B^0 Nice dream
eh?

Thanks for an informative post! Besides, I love happy endings..
Joe

From kee–(at)–ustin.ibm.com Thu May 16 16:53:14 CDT 1996
Article: 9832 of rec.audio.tubes
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From: kee–(at)–ustin.ibm.com ()
Newsgroups: alt.guitar.amps,rec.audio.tubes
Subject: Re: zener diode for dropping B+
Date: 16 May 1996 20:03:46 GMT
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In article , cign–(at)–elios.phy.OhioU.Edu (Dave Cigna) writes:
> Against many recommendations I bought a 56V, 50W zener and installed it
> in my Princeton Reverb. This is a stud mount device with DO-5 case that
> requires a 5/16 inch hole. I bolted it directly to the chassis without
> any heat sink. The HT center tap was lifted from ground and attached to
> the terminal of the zener.
Against what advice? It works great. As long as the zener is not over
dissipated, it’s almost a pure DC drop in the zenering direction, a diode
in the forward direction.
>
> With a GZ34 rectifier, B+ at the first filter cap dropped from 453V to
> 404V. The voltage on the plate of the first preamp tube dropped from
> 177V to 158V. There is a DC drop of 43V across the diode plus about
> 20VAC. The AC is due to the fact that the rectifier – and hence the
> transformer – is not conducting during the entire AC cycle. I bypassed
> the diode with a .1uF cap because it makes me feel better. If anyone
> can justify this, or explain why it shouldn’t be there, I am happy to
> listen.
What is across the zener is a the zero crossings when the AC is too low
to cause the diodes or zener to conduct, as you note. The cap is
unnecessary because nothing else is conducting during this time. An oscilloscope
across the zener should show you transitions from zero up to the zener
voltage (56V) where the zener starts conducting, a flat top at that voltage,
and then a transition back to zero when the zener turns off; this sequence
will repeat each half cycle of AC. Your meter reads 43VDC bacause it
averages the DC reading. It reads 20 VAC because it is reading that
as the average of the odd duty cycle AC waveform with the DC removed.

> I installed a toggle switch to short the diode so that I could A/B
> back and forth. I could find no additional ripple or hash on the B+
> with the zener in circuit. The cap didn’t change anything on the B+
> side of things, but it did shunt the AC at the center tap.
I would have not expected the diode or the cap to change anything except the
DC level at B+.

> This amp is cathode biased (a previous attempt to bring plate-to-
> cathode voltage down), so this modification didn’t affect the bias
> in any way. Amps that use a separate tap for fixed bias might need
> to have the bias supply reworked using one of the HT taps. I think
> that amps that are already using a HT tap for the bias supply will
> not require any changes.
>
> The diode gets warm, but it’s not uncomfortable to touch continuously.
> In this installation it is passing only about 50mA. It would certainly
> get hotter in a Twin Reverb, but I don’t think that a heat sink would
> be required if it is mounted to an uncluttered part of the chassis.
> Perhaps the back panel would be a good place.
Mount it where the current pulses running through it and through the chassis
don’t run through any part of the chassis used for signal grounds to
avoid hum problems.
>
>
> How does the amp sound? Great! Finally, I like to play this amp the
> way that I always felt that I should. The hard edge is gone and the
> springyness that I love about 6V6s comes through. This confirms what
> I have noticed before: 6V6 tubes just don’t sound their best with
> high plate voltages. They start to lose their 6V6-eyness with much
> more than about 400 volts. About 375V plate-to-cathode sounds best to
> my ears. In this case, the zener plus the cathode bias put it right
> on the money.
>
> Despite all the previous warnings against using a zener to drop B+,
> I am very happy with the results. Except for one small hole in the
> chassis, the mod is easily reversible. I may convert a seldom used
> Showman to 6V6s …
Good Mod.

R.G.

From cign–(at)–elios.phy.OhioU.Edu Fri May 17 08:54:51 CDT 1996
Article: 9867 of rec.audio.tubes
Newsgroups: alt.guitar.amps,rec.audio.tubes
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From: cign–(at)–elios.phy.OhioU.Edu (Dave Cigna)
Subject: Re: zener diode for dropping B+
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In article ,
William R Rodrick wrote:
>Dave Cigna (cign–(at)–elios.phy.OhioU.Edu) wrote:
>: Against many recommendations I bought a 56V, 50W zener and installed it
>: in my Princeton Reverb. This is a stud mount device with DO-5 case that
>: requires a 5/16 inch hole. I bolted it directly to the chassis without
>: any heat sink. The HT center tap was lifted from ground and attached to
>: the terminal of the zener.
>[etc.]
>
>Excuse my ignorance, but what is the polarity, and do you just insert the
>thing between the secondary’s center tap and ground? This sounds interesting
>(I have a Princeton, too, and from time to time think I have similar
>complaints
>about the sound with the regular “high” plate voltage).

Hi Bill, diodes with the correct polarity will have a cathode case. The
part that I found is NTE5278AK. I believe that the ‘K’ on the end refers
to the polarity. Mouser will sell them to you for about $14. They’re not
in the latest Mouser catalog, but they will get any NTE part for you if
you don’t mind waiting a week or so. There are similar parts available
in voltages other than 56V, but I don’t know the numbers.
Mouser: 800-346-6873 or try http://www.mouser.com

Yes, they are just inserted between the center tap and ground. After
bolting the diode to the chassis just unsolder the CT from the ground
terminal and solder it to the terminal on the diode.

R.G. Keen made a good suggestion about locating it on the chassis and
I’ll repeat it here. Remember that the current will flow through the
chassis from the diode to wherever the filter caps are grounded. Also,
the audio signal flows through the chassis from wherever there are signal
grounds to wherever the filter caps are grounded. In order to keep hum
to a minimum you want to avoid placing the diode in the path that the
signal takes, or vice-versa.

>Also, what precautions are necessary when using fixed bias? Are you just
>referring to the necessity of readjusting the bias after doing this mod,
>or are more drastic measures called for?

When the zener is used in the center tap of a secondary winding the
voltage at every tap on that winding will drop by the zener voltage. Many
amps have a tap on the HT secondary for the bias supply. If this tap
normally provides 60V, then with a 50V zener in place the bias tap will
only provide about 10V — not enough for a bias supply. The fix is to
use one of the HT leads on the same winding. These are at ~300V instead
of 60V, so you need to swap the resistors in the voltage divider in the
bias supply. It’s not very difficult if you know what you are looking
at. Some amps (like a stock Princeton) use the HT winding to derive a
bias supply anyway. It should not be necessary to make any changes to
these although I would suggest checking and adjusting the bias if
required after installing the zener.

— Dave Cigna

From greg–(at)–ud.lattice.com Thu May 23 12:40:37 CDT 1996
Article: 10076 of rec.audio.tubes
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From: greg–(at)–ud.lattice.com (Grego Sanguinetti)
Newsgroups: alt.guitar.amps,rec.audio.tubes
Subject: Re: zener diode for dropping B+
Date: 23 May 1996 01:15:35 GMT
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>Any low-level zener noise should be filtered easily in your
>configuration.

Yes I think that his .1uF cap across the zener should be sufficient
to shunt the zener noise.

very reasonable mod actually. I like it. 340-380v is where I typically
pp 6V6 guitar amps. All the Fenders are too high in stock form for
my tastes. A matter of opinion of course.

-grego

————————————————————————–
Grego Sanguinetti, Lattice Semiconductor Corp. | Water, water everywhere,
greg–(at)–atticesemi.com | but I’d rather drink beer

From Rlockye–(at)–tbbs.com Thu May 23 12:40:58 CDT 1996
Article: 10082 of rec.audio.tubes
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From: Rlockye–(at)–tbbs.com (Rich Lockyer)
Newsgroups: alt.guitar.amps,rec.audio.tubes
Subject: Re: zener diode for dropping B+
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>Yes I think that his .1uF cap across the zener should be sufficient
>to shunt the zener noise.

>very reasonable mod actually. I like it. 340-380v is where I typically
>pp 6V6 guitar amps. All the Fenders are too high in stock form for
>my tastes. A matter of opinion of course.

I’m planning on building a 6V6 amp, and would like to run B+ around
300-325. The problem is that I can’t get a power transformer under
275 or 300, and I really need something in the 240-250 range.

It looks like a 50-60v Zener will do the trick. What kind of wattage,
or specifically, what part number (preferably ECG) would I need?

This is something that I’ve never thought of. People have suggested a
voltage-divider network, which of course, wastes power and will make
B+ highly load-dependent. Thanks for the idea guys!

From kee–(at)–ustin.ibm.com Thu May 23 12:41:31 CDT 1996
Article: 10101 of rec.audio.tubes
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From: kee–(at)–ustin.ibm.com ()
Newsgroups: alt.guitar.amps,rec.audio.tubes
Subject: Re: zener diode for dropping B+
Date: 23 May 1996 15:50:23 GMT
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In article <4o13ka$jm--(at)--ews.paonline.com>, Rlockye–(at)–tbbs.com (Rich Lockyer) writes:
>
> I’m planning on building a 6V6 amp, and would like to run B+ around
> 300-325. The problem is that I can’t get a power transformer under
> 275 or 300, and I really need something in the 240-250 range.
>
> It looks like a 50-60v Zener will do the trick. What kind of wattage,
> or specifically, what part number (preferably ECG) would I need?
>
> This is something that I’ve never thought of. People have suggested a
> voltage-divider network, which of course, wastes power and will make
> B+ highly load-dependent. Thanks for the idea guys!
Zeners are like any other electronic component. The power dissipated is the
time integral of the product of the voltage and current through them. In this
case, the power is just the product of the zener voltage and the amp current,
allowing for the current to be higher at max volume than it is at idle.

Take the bias current you’re going to run, probably 20-30ma for a pair of
6V6, double it to, say, 60Ma, and multiply to get 3.6 Watts. That’s what it
would be if the current was DC. Since there is a high multiplier for the RMS
to average in the rectifiers, the actual RMS current could easily be double that
again, for 7.2W, so a 10W zener would work if you could get enough heat sink
on it. For margins for heatsinking, and since a 20W or 30W zener is not much
more expensive than a 10W, get a 20 or 30W, and have a lot of margin.

Be sure to heat sink it some way, and get the right polarity on the case for
tying the case directly to the chassis, so you can heatsink it to the chassis and
not have to insulate it from the chassis electrically.

I don’t have an ECG catalog to look up the part number.

R.G.

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