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From: Mark Beckles Willson
Date: Sun, 29 Jun 1997 09:52:45 +0100
Subject: Re: 6V6 output impedance

At 08:41 PM 6/27/97 -0500, you wrote:
>Don't know amp-l, so hope y'don't mind if I ask some of the
>amp-mavens on harp-l.

alt.guitar.amps might be the place to ask.

>My Premiere amp blew its output transformer, so I bought one
>from Hammond Mfg.Co., their model 125C. With six contacts,
>you can hook it up every which way from Sunday. The Premiere
>is a type A, with a single 6V6-GLT output tube. An ohmmeter
>across the disconnected single 8" speaker registers 3.2 ohms
>of DC resistance.

>Here are a few of my choices to connect the transformer:
>
>Voice Coil Primary impedance, from the 6V6 (ohms)
>Impedance
>(ohms) a b c d e f g h
>
> 3.2 18000 11500 9000 7000 5100 4600 3300 2700
>
> 4.0 22000 15000 11200 8800 6400 5800 4100 3400
>
>Now my questions:
>
>1) What is a 6V6-GLT's "book" output impedance? Where on the table?

The output transformer primary impedance which a single 6V6 likes to see
varies with the anode voltage (in simple terms). At an anode voltage of
250V it should be 5k5 ohm and at 315V 8k5 ohm according to the RCA
receiving tube manual. Fender ran 6V6s at anything up to 420V in some
amps, way off the recommended scale, but I have no idea what voltage
Premier used.

>2) Does a DC resistance of 3.2 ohms mean 3.2 or 4 ohms impedance?

The speaker is almost certainly 4 ohm impedance.

>3) Could my transformer have failed because of some other problem,
>e.g. old capacitors, that may also burn out the Hammond transformer?

Yes. A shorted power tube is the most likely but it could be a bunch of
other things.

>Also, Hammond includes a note:
>
> When operating at impedances below nominal, power capatility
> and the frequency spectrum are propotionately lower; conversely
> at higher impedances power capability and frequency spectrum will
> be proportionately higher".
>
>4) What impedance are they referring to? Speaker or Primary?

They are refering to speaker impedence. The increased power and bandwidth
running into a higher impedence is theoretical rather than practical in my
experience.

>5) If I wanted to experiment for a better harp sound, which way do
> you think might give a nice sound? Above or below nominal?

I suggest that you leave your amp's impedence match as close to stock as
possible. This is a big area for snake oil and black magic in the amp
world and opinions vary. In order to mismatch impedences safely, however,
you need to have some knowledge of the conservatism of the amp and
transformer design. Thinking about older amps, for instance, I should be
much happier running a Fender into the "wrong" speaker impedence than a
Marshall....

Finally, and with all due respect John, I am worried about your trying to
replace the output transformer in your amp if you have to ask these
questions. You run the risk of seriously damaging yourself, your new
transformer and your nice old amp. I suggest that you take it to someone
who knows about valve amps. On the other hand, if I'd always followed that
advice, I wouldn't know anything about them. As a minimum, however, you
should read one of the recent books about repairing valve amps,
particularly the section on safety, before attempting the work.

Mark