Bias too hot when tremolo intensity increased

[daveneary]

Hi, 
  I have put together a Deluxe Reverb and Biased it at approx 22 ma. The bias voted is about -36 volts. And plate voltage about 424. Using JJ's. Sounds as expected

When I turn the Tremolo intensity up the current draw goes to 130 ma as the bias voltage goes down to -20 ish.

 If I measure the voltage at the intensity pot lugs with the tremolo intensity on minimum  I get  -35,36 volts on the bias side lug and the wiper lug. The other lug from the LFO reads between 25 and 55 volts as it oscillates.


With the tremolo intensity increased to 4 the bias varies between -19 and -25 V with current draw 56 to mid 60's. At maximum I get a  tremolo effect but noisy and the current draw >100 ma which is beyond healthy :-)


I thought the tremolo should increate the bias voltage to turn down the valves but mines appears to work the other way. Any thoughts ?

[sluckey]

Is this the same amp you built over a year ago? Did you use the Hoffman single channel AB763 plans? Did the trem ever work properly? Tell us about any mods you have made to the tremolo circuit.

A properly wired Hoffman circuit with good components doesn't behave like you describe.

[daveneary]

Hi ,
  thanks for the response.  This is the same one - I built range of amps at the time ranging from Champ, 6G2, Princeton Reverb and the Deluxe. 

I was never happy with the noise level on the Deluxe despite lead dress, alternative power transformer. The tremolo was working but intensity was not strong enough. To attempt to address this -  I swapped the 470K resistors in the LFO to 330K.  To get the noise level down to acceptable levels  - I biased the power tubes low  < 10ma which made the amp quiet. I also added a SPST switch to make the Trem switchable. When I engaged the tremolo in this state it worked but introduced distortion and more noise.  I thought that would be because the cold biasing + bias variation from tremolo circuit was effectively turning the power tubes off.

I just recently started looking at the amp again - discovered I had not added the last capacitor in the power supply which may have accounted for some of the noise.  Having done that I started to look at the biasing circuit and found another issue which I corrected as per Hoffman One Channel schematic/layout.

I rebiased to the values from my last post  and removed the trem. switch.  The overall sound is now good with tremolo at minimum. With the tremolo intensity raised, the tremolo works but there is more noise and the values are per previous post.
 
As I mentioned - I thought the trem operation normally increased the bias voltage.

Dave.

[jjasilli]

??? You should post schematic.  Deluxe reverb has opto trem > PI.  Sounds like you are using bias vary trem > power tubes. 


Trem LFO is a varying voltage from: -x to -0- to +x volts.  For power tube bias vary trem, this causes the bias voltage to decrease and increase successively.  Within a proper range, the temperature variation of hot > cold bias will average out.  The worst problem is redplating if the bias gets too hot (Bias voltage too low). 


So, per your 1st post, your trem vary bias should vary up & down with -36VDC as the center point, measured on the power tube Grids.  But it's dropping down to a a swing of -19 > -25.  I think the swing seems narrow, only +/-3VDC, and around the wrong center point.  Something is draining away some of the bias voltage with the trem ON. 


Also the trem should not add noise which suggests a bad or un-grounded cap(s) in the trem circuit.

[daveneary]

Hi  jjasilli,
Thanks for your suggestions.
 
 using the Hoffman single channel bias vary tremolo schematic from the site.  I have fitted into a Princeton Chassis drilling an extra hole for extra 12AX7 tube.  I think the extra noise is just the power tubes being way too hot biased. Do you have any ideas what +x and -x values should be and what is the best way to measure it. I am assuming at the pot lug and chassis earth.  When I do that the values are all positive but vary from 20 to 50 odd volts. When I increase the intensity the voltage falls to the range described and the current draw is 100+ ma and red plating.  I would have thought the bias would stay centred at -36 volts varying between -30 and -42 volts.

Regards.
Dave.

[jjasilli]

See:  http://www.lynx.net/~jc/tremoloCircuits.html


For power tube trem I seem to remember +/-5VDC for a total swing of 10VDC.  Maybe someone can check me on this. 

[daveneary]

just disconnected the wire from .1 uf to the intensity pot lug.  The voltage swing is about 6-10 volts but at about 100v + - 5 V.

must have a bad value somewhere.

[sluckey]

Replace that .1µF cap that connects to the pot and see if it behaves better.

Refer to the schematic on page 4 of this pdf. It's exactly the same tremolo circuit as you have, but the power amp is a bit different and uses a different tube. Operation is the same as your amp though. Here's the link...

     http://sluckeyamps.com/hammond_2/AO-63.pdf

There are some notes and voltage readings on that schematic that should be helpful to you.

Do you have any ideas what +x and -x values should be and what is the best way to measure it. I am assuming at the pot lug and chassis earth.  When I do that the values are all positive but vary from 20 to 50 odd volts.

You should never see a positive voltage on any lugs of the intensity pot. This is why I suggest to replace that .1µF cap.

I just made some checks on my amp (see schematic above). I measured the voltage across the 10Ω cathode resistor for V3. My readings will be ten times what you would get on your 1Ω cathode resistors. I had my tremolo speed set to 5HZ for these readings.

1. With Intensity set to zero, my bias test point for V3 read 250mV (represents 25mA)

2. With Intensity set to MAX, my bias test point voltage jumped around as expected. Luckily, my meter has a MIN/MAX/AVG button. I just put the meter into min/max mode and waited. The meter beeps every time a new min or max value is sampled. After about a minute the meter quit beeping because there were no more new min/max values being sampled. Then I just read off the min, max, and avg voltages recorded.

MIN was 83mV (represents 8.3mA)
MAX was 690mV (represents 69mA)
AVG was 359mV (represents 35.9mA)

At no time were the output tubes redplating. This is typical of the operation of this tremolo circuit.

Maybe this info can be of some use to you.

If you can post some hi rez pics of your amp guts, concentrating on the trem circuit components maybe we can help. Also, disable the trem with the footswitch and measure the voltages on all pins of the trem tube.

Is your amp working properly with the trem turned off?

[daveneary]

Hi
   I have rebuilt the tremolo section of the deluxe reverb replacing the 0.1 uf cap.  Amp now sounds great.  Will attach a couple of photos showing the rebuild. I think one issue was the C22 capacitor on the hoffman's schematic was 0.1 uf rather than 0.001 uf.  I only had a 0.002 uf available so not sure of the impact of that but at least within range.  I upped one of the caps in the LFO to 0.02 to slow down the tremolo and had to lower the bias to get the intensity  I wanted. Amp now sounds good with a working tremolo.  I had previously modded the tone stack but will put it back to standard and go from there.

Thanks to sluckey and jjassilli for advice and help.

Regards
Dave.

[jjasilli]