OT Logic

[Markusv]

Please help me check my logic here

I have a Bogen 50 watt PA that already sounds pretty decent as a guitar amp.
But I want to tweak a bit and play around with speaker loads, tonestack tweaks and so forth

The OT has the following resistance readings:

Primary from blue to brown = .222K
Secondary (supposedly 8 Ohm winding) .6R

So that gives me a ratio of 370

Given the fact that the amp runs on a pair of 6L6's (PP) and the voltage at the OT Center tap is around 420VDC -please help me understand what plate impedance I'm working with?

Do I take the Plate- to -plate impedance of that pair of tubes and use the ratio to figure out the speaker load?
My math and general grasp of this area is limited at best
Please bear with me- but I would appreciate some clarity from anyone who is willing

Markus V

[Firestorm]

You can't use resistance to figure the turns ratio. You need to input a known voltage AC signal in one side and measure the voltage out the other. Calculate turns ratio from that. If you don't have a signal generator, you can use a 6VAC wall wart in a pinch, but it won't be completely accurate since transformers are typically rated at something like 1000Hz rather than 60Hz. You need a good meter, too. Preferably two.

[Cliff Schecht]

I would simply assume the plate resistance is anywhere from 4k to 6.6k. Probably closer to the latter, 6L6's like a relatively high plate resistance (compared to higher output tubes) and surely the lazy designers at Bogen knew this.

[Markusv]

Thanks Firestorm and Cliff

So to take it a bit further
Lets assume the Primary is 6.6K
And let's say I want to mod the amp to something that has a 4K primary in the schematic.

Do I simply alter the speaker load so that the ratio between primary and secondary reflects the lower primary number?
Meaning that - I would have run an 8 ohm speaker load in the past- but now I would choose a 16 ohm load instead (for arguments sake) but sill use what was the original 8 ohm secondary tap?

Markus V

[Firestorm]

The windings on an OT are typically tapped in increments of 2 (2 ohms, 4 ohms, 8 ohms, 16 ohms) so using different taps will also change reflected impedance only in increments of 2.

If you want to know how it works: a transformer having a 6600 ohm primary into an 8 ohm tap, has a turns ratio of 28.7228 (6600/8=825; sqrt825=28.7228). Into the 16 ohm tap it is 6600/16=412.5; sqrt412.5=20.31. Into the 4 ohm tap, it's 6600/4=1650; sqrt1650=40.62.

To calculate the effect of different loads on the primary impedance, you square the turns ratio and multiply by the intended load. So in your hypothetical example (putting 16 ohms on the 8 ohm tap), you take the turns ratio for the 8 ohm tap (28.7228), square it (825) and multiply by the load (16). So you get 825x16=13,200. Not what you intended. Going the other way (putting 4 ohms on the 8 ohm tap) you get 825x4=3300. Lower than your target.

The only way to get close to your 4000 primary is to use a speaker load that isn't 4, 8, or 16. 825x4.8484=4000, so you need a 4.85 ohm speaker. The closest you'll get is with 3 parallel 16s. There the load will be 5.33, so 825x5.33=4397.25. Close.

[Markusv]

Firestorm

Great and useful explanation

Thank you!