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    Building, troubleshooting and testing of these amplifiers should only be
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Are these OT Winding Ratios correct?

Greetings,
I'm getting ready for my first tube amp project ever, although it might be challenging I'm trying to build a guitar amp inspired from a Mesa Boogie (for personal use).

I bought a new transformer rated for the original amplifier, which supports 2x 6L6 + 2x EL34, with the following specs:

75W max. Raa ca. 3.6k
M6 Lamination: 3,75", Stacking: 1 9/16"
Mounting on 79mm x 54mm

Primary: Orig. colors in brackets
A1 6L6 Blue (Blue)
A1 EL34 Violet (Blue/White)
B+ Red (Red)
A2 EL34 Orange (Brown/White)
A2 6L6 Brown (Brown)

Secondary:
0 Ohm Black (Black)
4 Ohms Green (Green)
8 Ohms Yellow (Yellow)

As you can see the OT has two separate taps for 6L6 and EL34.

Since I don't want to destroy anything, I decided to do the math/simulation first and test the output transformer by connecting a variac on the primary and measuring the unloaded voltage on the secondary (8 ohm tap), until the secondary voltage reaches exactly 1V (as suggested by Uncle Doug on a YT video).

The values that I measured are:
For 6L6: Brown-Blue: 19.1V, Black-Yellow: 1.0V
For EL34: Violet-Orange: 15.27V, Black-Yellow: 1.0V

It gives an primary impedance of ~2918 ohm for the 6L6 tap and 1865 ohm for the EL34 tap, using the formula (8 ohm)*(Wratio^2)
Are these values compatible with a PP of the given tubes? I usually read much higher primary impedence values used for those tubes..

The specs say that it should have a Raa of about 3.6K, but my values are quite different. What am I overlooking? Perhaps should I have measured it using an 8ohm load?
 
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If your transformer was designed for a guitar amp, then it may not handle very well 50/60Hz. I'd recommend loading the secondary with 8ohms as you suggested, and repeat the measurement, because you will always get a higher output voltage when the transformer is not loaded.
 
Welcome to Tubes / Valves threads, mostly about Hi Fi / Stereo amplifiers.

Perhaps someone on Instruments and Amps threads has 6L6 or EL34 guitar amplifiers with those exact transformer ratios.

Often, guitar amplifiers and Hi Fi amplifiers are designed with quite different goals for the kind of sound that they put out to the speakers.
 
I bought a new transformer rated for the original amplifier, which supports 2x 6L6 + 2x EL34...
Does this mean that this amplifier features a pair of 6L6's and EL34's (four output tubes) at the same time? Then I'd regard your findings as correct.

Btw, I always connect the primary winding of an unknown output transformer to the mains voltage, using an isolation transformer, of course. The secondary voltage measurements are more precise then.

Best regards!
 
Depanatoru,

Interesting . . .

The EL34 has higher transconductance than the 6L6, so perhaps they divided Both the Bias voltage, and the Signal voltage to the EL34 to make its quiescent current be approximately the same as the 6L6 quiescent current.

The EL34 in triode wired mode has less signal gain, versus the 6L6 in beam power mode signal gain.

They divide the drive signal voltage to the [lower gain] EL34; but they do not divide the drive signal voltage the [higher gain] 6L6.

It seems that they want the 6L6 to provide most of the signal current 'to' the output transformer; but the lower gain [triode wired] EL34 has lower rp than the 6L6, so the EL34 is used to increase the damping factor.

Not my cup of tea, but I bet Marketing and Sales Loved It!

Just my opinion
 
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As I don't see any technical reason, I strongly suspect that Mesa Boogie did this design just to be different and to charge even more $$$$ ;) .
Hi Kay! Guitar amplification has totally different goals compared to hi-fi amplification.
Just to name a few:
  • you want an amp that has the possibility to have strong distortion at low volumes, and keep them at high volumes as well;
  • you want screens' voltages to drop to have compression during peaks;
  • you want the damping factor to be very low, because the increase in impedance of the speaker at low and high frequencies are used to equalize the sound by removing specific frequencies from the nfb loop;
  • etc...

That's why that amp could work with EL34 in triode only (distortion at low volumes) or add a pair of 6L6GC in parallel to push more volume.
I've had one of those amps (the Mesa Boogie 295) and it was one of the best commercial amps I've had.
 
It seems that they want the 6L6 to provide most of the signal current 'to' the output transformer; but the lower gain [triode wired] EL34 has lower rp than the 6L6, so the EL34 is used to increase the damping factor.
Almost that. The amp could be used with EL34 only to have what in guitarland is called "a bluesy sound": alot of low order distortion at low volumes. Then, if more volume or a different sound was needed, you could switch on the pair of 6L6GC to increase the wattage of the amp.

That amp was indeed declared as 35 W with EL34 only, and 85W with EL34+6L6GC.
Damping factor is always well below 1 in all guitar amps for the reasons I explained in the previous post.

Just to give you an idea, all Queen guitar sounds you hear are played on an amp without gnfb, with a quad of EL84 cathode biased in PP pentode on a 4k load.
You can imagine how a different speaker becomes crucial in the final sound you get.
 
Hi Zintolo,

I've heard that the core of Brian May's (guitarist for Queen) signature sound is a small transistorized amplifier that bass guitarist and EE Jogn Deacon built for him from parts of an old radio. Maybe that May somtimes fed this amp's output signal into another tube amplifier (Vox AC15 or AC30).

Best regards!
 
I've heard that the core of Brian May's (guitarist for Queen) signature sound is a small transistorized amplifier
That's not quite true.
He basically used AC30's with a treble booster.
Plenty of resources available for that, but he talked about this in an interview with Rick Beato:

He talks about his setup around 18:22

Later in the video they also about the little Deacy Amp, that's only being used in some songs.