Deluxe Reverb Single Channel

[daveneary]

Hi,
   I'm trying to power up a one channel Deluxe Reverb for the first time. Haven't connected the B+ or any tubes yet. Just trying to get the bias voltage.  The power transformer is sourced in the UK. It has 2 Greens (heater)+Yellow/Green Centre Tap, 2 Yellows(5v Rectifier Heaters), 2 Red +B supply.   I notice on the Schematic for the One Channel DR , there is expected to be a tap for the Bias tap. My power transformer does not have one. The voltage drop resistor prior to the diode is 470R 3Watt. If I am taking my Bias directly from the AC feed to the rectifier - what will my resistor need to be ? I used a 3W 100K doing the same for a Princeton clone which gave me a usable -20 -65 range with a 10 k linear bias pot.

With the 407R 3 watt -  I am getting  -150 volt and a smoking bias pot :-)

Any ideas.

[sluckey]

Change the 470Ω to 100K, just like the Princeton Reverb.

You may have smoked the filter cap too. What is the voltage rating for the filter cap(s)?

[eleventeen]

When taking bias off the HV winding, meaning, no separate bias tap, the various resistors want to be in the 10's of K-ohm range. Not 500 ohm (eg; not 470 ohm)


I always have to fiddle with these values, thus I tack them in place, usually using junkbox parts and hanging them in the air to get approximate values before I nail anything down or mount them neatly. Usually something like 15K >> 50K ohm pot >> 22K....or, 33K ohm>> 50K ohm pot >> 12K...the point being, the entire voltage divider should add up to something in the area of 75K - 100K ohms.


Remember as well, that if you are building your bias supply BEFORE building your main power supply, that when you finally get the load of the output section onto the supply, it will be dragged down in volts some. 

[daveneary]

Thanks for the advice. 50K + 8K pot  sorted it at least to power it up. Getting a range of 15-45 mv across the 1ohm resistor although  both 1ohms resistor read 1.3. Anyway left at 25 ma ish for now. Sounds good although barely  perceptible tremolo.

A,B, C and D voltages read 415v, 412v, 404v and 379v respectively

and both power tubes have plate voltages of 405/6, and -26V bias voltage
 
Assuming  JJ 6V6S are rated at 14 Watts     % = (.025 * 406 )/ 14 --> 72.5%

Any thoughts on the tremolo - is it a bias issue ?

Regards
Dave.

[sluckey]

Sounds good although barely  perceptible tremolo... Any thoughts on the tremolo - is it a bias issue ?

I assume you're using the bias vary tremolo circuit found in Hoffman's amps? If so, yes, the bias is a factor with the tremolo. You'll have to find a compromise between bias and trem operation. Experiment with the bias pot to see if you can find your happy spot.

There are several easy mods you can do to increase the strength of the tremolo. The most effective mod is to replace the oscillator cathode resistor/cap with a 5mm red LED.

[daveneary]

Hi
Thanks for the reply. Poor solder on one of the caps. Can get a decent tremolo now but at pretty low bias less than 20 ma. I seem to recall there were some component changes to give a deeper intensity. Will play for a couple of weeks before doing any more tweaks to check reliability etc.
Dave

[eleventeen]

"although  both 1ohms resistor read 1.3. "


Just as a FYI, this is pretty typical...you're reading the resistance of your test leads in series with the 1 ohm R.


Carry on!