Eric Tremolux

[tubeswell]

Got an EC tremolux on hand at the moment and had a go at tracing out the schematic. The amp is based more on the BF Princeton with a vibrochamp trem, than any original tweed tremolux. And yes, they have rectifier protection diodes.

(Edit updated schematic added)

[tubeswell]

updated png version

[tubeswell]

sch version (for hackers)

[shane]

Hi Tubeswell
Nice one mate! What's the bias pot value by the way?
Cheers
Shane

[tubeswell]

Forgot to measure the bias pot! - I'll have to open it up again

Edit

Bias Supply Voltage: -38.1VDC at supply, -20.5V at junction of bias tail resistor and pot (53.8% of Vin) therefore, assuming 220k bias tail resistor, the bias pot calculates at 190k (200k). (Bias voltage was -30.7VDC at grid, which is about the middle of the pot range)

[pdf64]

Fender have been happy to email schematics if asked, did you try that?
I can forward the one for the EC Tremolux if you'd like, though your's looks to be spot on.
The bias pot is noted as being 150k!

[M Fowler]

I would like to see the Fender Schematic mjensenfowler at aol dot com
if you don't mind.

The 1/4 power reduction is interesting.

Pete where are those 25w power reduction resistor located I don't see them in photos? On back side of chassis?

Mark

[pdf64]

The 1/4 power load resistor is noted as being chassis mounted.

What happens to the bias supply voltage when in standby (PT HT CT is lifted)?

[M Fowler]

The 1/4 power load resistor is noted as being chassis mounted.

Thanks for the information.

Mark

[tubeswell]

The 1/4 power load resistor is noted as being chassis mounted.

What happens to the bias supply voltage when in standby (PT HT CT is lifted)?

That's the crazy thing IMHO. The bias supply voltage completely disappears when the standby switch is off. Pretty risky methinks.

I think a 'better' improvement would be to move the standby switch to the B+ rail (or remove it altogether) and have a BFPR type of bias supply in this amp. The way it is in the stock fender configuration, its difficult to get a more negative bias voltage if you need it. After 38VDC there is nowhere to go, and the B+ is quite high.

The pictures show readings before retubing and rebiasing. (Deducting ~2mA for screen current) V4 is idling at 10.7W, V5 is idling at 12.3W. Even after winding the bias pot to maximum negative voltage (-38.1VDC), I could only get V5 down to 10.5W. I didn't bother modifying the bias circuit - just put new tubes in (JJ6V6S) - and I could get these to idle at 76%, and still have rotation to spare on the pot.

[tubeswell]

I would like to see the Fender Schematic mjensenfowler at aol dot com
if you don't mind.

The 1/4 power reduction is interesting.

Pete where are those 25w power reduction resistor located I don't see them in photos? On back side of chassis?

Mark

Hi Mark - See pic

[tubeswell]

The bias pot is noted as being 150k!

Thanks for that

[Colossal]

Thanks for posting this Tubeswell. So that's where they are hiding those power resistors for the power switch.

I found a demo online of the amp (with good recording) and it sounds quite good with the Celestion Heritage G1265. The demo was played with a Tele with Joe Barden twin blade pickups. It sounded very full with good bass. Have you tried the amp with a full humbucker? It has 0.1uF coupling caps in the power section and I'm wondering if the preamp is pushed, if it will flub out. I usually lower the coupling caps and mod the cathodyne to stop that. I hate that skidding, ill defined, rubbery mess a Tweed can produce when overdriven hard.

[tubeswell]

Yep - it starts to break up earlyish (like around '5'), and with over biased little 6V6GTs running flat out with less than optimum bias voltage, the grid current from the output tubes tends to get to blocking distortion quite easily. The 1k5 grid stoppers don't do anything.

With JJ6V6S, it at least biases up properly. The plates are sitting at ~410V@26.1mA (10.6W), which is okay for a 14W tube, and the customer is very happy with the sound. (Whereas previously, the old 6V6GTs running hot were making funny sub-harmonic distortion noises as it got louder)

I daresay you could get away with a 20W rating for the attenuator resistors, because when in use, they are hooked up in combination with the speaker'n'all. The 16R sees half the power of the secondary load, the 8R and the speaker together see the other half the power. Half of 12W is 6W, etc.

[John G]

Hi,
Is this the same Animal.
John