[Mark Robins]

I recall seeing info once that PA line matching transformers could be
used for speaker matching in guitar amps.

Can anyone elaborate on the pro's and con's of this relating to both
single ended and parallel configurations?

Given a certain level of compromise, are there other or better design
alternatives that do not require the use of expensive OT's?

Thank you,

Mark Robbins

[Patrick Turner]

Mark Robins wrote:

I recall seeing info once that PA line matching transformers could be
used for speaker matching in guitar amps.

Can anyone elaborate on the pro's and con's of this relating to both
single ended and parallel configurations?

Given a certain level of compromise, are there other or better design
alternatives that do not require the use of expensive OT's?

Thank you,

Mark Robbins

The line matching trannies are all ungapped trannies which normally
have far too few turns per volt to allow hook up to any tubes except
perhaps to low Ra trioders such as 6AS7.

They are all mainly made to connect multiple speakers to a "100V line"
from a solid state amp
so many speakers around a factory, shop or school can be wired without
using thick copper wires on long runs which would have far too much
inductance
to maintain the HF response.
At each speaker there is a 100V to say 10V step down tranny,
so that the tranny is actually a 100 : 1 Z ratio, so about 800 ohms to 8
ohms.
Usually such trannies are crude and not well interleaved and have high
winding resistances
unless they are toroidal, in which case then they can be wide bandwidth.
Its no good to try to hook up a pait of KT88 to such a tranny.

But were you to have say 10 x KT88 in parallel SET with a choke feed for
the DC supply,
then you could cap couple to a line tranny with 600 ohm input impedance
and say 100 watt rating. The load seen by each KT88 will be 6,000 ohms,
about right,
and the primary is coupled via a suitable cap of about 100uF mimimum,
with one end of the primary grounded.

Because no gap is in a line tranny, and there is usually no primary CT,
then their use in normal configurations in tube amps is not
usually feasable.

Patrick Turner.

[Phil Allison]

"Mark Robins

I recall seeing info once that PA line matching transformers could be
used for speaker matching in guitar amps.

Can anyone elaborate on the pro's and con's of this relating to both
single ended and parallel configurations?


** Do you know what a Google Groups Advanced Search is ??

http://groups.google.com.au/group/re...ad98cc e8dbc2



Given a certain level of compromise, are there other or better design
alternatives that do not require the use of expensive OT's?



** Since 70 / 100 volt audio line and AC power transformers are NOT
designed for push pull operation and also cannot accept the DC current of a
class A stage - you have a problem.




......... Phil

[Patrick Turner]

flipper wrote:

On Sat, 14 Oct 2006 05:38:55 +0930, Mark Robins
wrote:

I recall seeing info once that PA line matching transformers could be
used for speaker matching in guitar amps.

Can anyone elaborate on the pro's and con's of this relating to both
single ended and parallel configurations?

Given a certain level of compromise, are there other or better design
alternatives that do not require the use of expensive OT's?

Thank you,

Mark Robbins

Well, it's been done for a modest 10 watt 'stereo' amp, and he seems
pleased with it, but I don't know if it could handle sustained power
like a guitar amp.

http://home.alphalink.com.au/~cambie...mp/6an8amp.htm

I breadboarded up one using PP 6EM7s and a Speco 70 volt line
transformer and it worked 'decent' enough that I'm building a whole
one, triode mode 6GK6s this time, just for the fun of it but I
wouldn't suggest it as anything more than junk box amusement.

I've also breadboarded a 20 Watt guitar amp using a power tranny I had
lying around as OPT but it's not really much, if any, cheaper than a
cheap 20 Watt guitar OPT if you have to buy it, and especially not
when you consider the total cost of the whole amp. Like, say, this one

http://cgi.ebay.com/Deluxe-Output-Tr...QQcmdZViewItem

(I couldn't get http://www.triodeelectronics.com/ to come up)

You can forget SE into either a line or power transformer because
they're not gapped and PP requires precise bias balancing (I'm using a
one sided servo).

You missed the point I made about line trannies and SE amps.

It is quite possible to use a line tranny to connect to any SE amp
if the SE amp's DC flow is brought to the tubes via a choke
which allows the primary of the line tranny to be coupled via a cap to the
anode/cathode circuit of the tubes.
Its the parafeed connection. DC gapping is thus not needed.

PP operation is also possible where you have two tubes in series
with DC flow from a B+ supply through both to 0V, and
with top tube cathode connected to bottom
tube anode. Output is via a cap to the OPT which again does not need a gap or a CT.
The arrangement is very similar to the topology of OTL amps based on Futterman circuits.
The Circlotron circuit can be employed to allow PP use of tubes in an OPT where the DC flow
is through one winding in both directions, and no gapping or CT is needed.
Since the line trannies have such low primary load values in hundreds of ohms rather
than thousands of ohms, tubes such as the 6AS7/6080 can be used but far better are tubes such as the
6C33C which is used for OTL and which will power into loads of say 500 ohms
to give excellent class A performance which is better than the nearly class B performance
used in OTL amps which requires large amounts of NFB
to make them anywhere near as linear as a normal transfromer coupled amp with proper
load matching.

Altronics supply 150 watt toroidal line trannies which have wide BW and which would be fine
for a pair of 6C33C.

For guitar amps the idea is to have conservative load matching to allow the
tubes to be grossly overdriven without their Pda rating being exceeded.
OTL amps are thus quite hopeless as musician's amps and are never used, but using a line
trannie with an OTL amp makes the tubes loading benign and Pda much lower so
such OTL amp topologies could be tried by a musician.
Whether it sounds as good a s apair of 6L6 and conventional deign is a moot point.

The trend now is towards SE class A guitar amps with low power, and they are simply miked up
and the sound amped up with thousands of solid state amp watts.
There is a trend away from musicians having to lug huge heavy amps into venues
where a "house sound system" is a permanent facility.

Patrick Turner.

[[email protected]]

On Sat, 14 Oct 2006 05:38:55 +0930, Mark Robins wrote:

I recall seeing info once that PA line matching transformers could be
used for speaker matching in guitar amps.

I've tried it in a 1 watt amp...

Can anyone elaborate on the pro's and con's of this relating to both
single ended and parallel configurations?

They can't take much DC so forget parallel... or high power... or wide frequency
response. (They sound like ****!)

Given a certain level of compromise, are there other or better design
alternatives that do not require the use of expensive OT's?

Sure, buy cheaper transformers... or salvage used equipment... or wind your
own... or use enough tubes to direct couple... or use a resistive load or a
choke and a capacitor...

Thank you,

Mark Robbins

[Mark O'Polo]

On Sat, 14 Oct 2006 05:38:55 +0930, Mark Robins
wrote:

I recall seeing info once that PA line matching transformers could be
used for speaker matching in guitar amps.

Can anyone elaborate on the pro's and con's of this relating to both
single ended and parallel configurations?

Given a certain level of compromise, are there other or better design
alternatives that do not require the use of expensive OT's?

Thank you,

Mark Robbins

Yeah - you can also use filament trsformers - like 6V and 12V ones as
output transformers. It may not sound as good or run as efficiently,
but in some cases they will have a close enough match they can
sound just as good as a tranny intended for Loudspeaker
Output service.