Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on March 21, 2011, 01:06:31 pm
-
I am converting a Princeton Reverb RI to a Point to Point PR.
I'm almost done. Added variable fixed bias and a midrange control on the back.
IMO the Tremolo is a bit weak. What's the best way to increase the strength?
Schematic...
http://www.el34world.com/charts/Schematics/files/fender/PRINCETON_REVERB_AA1164.pdf (http://www.el34world.com/charts/Schematics/files/fender/PRINCETON_REVERB_AA1164.pdf)
The Trem does pick up strength when I remove the NFB but that's just another option to add.
Thanks!
-
IMO the Tremolo is a bit weak. What's the best way to increase the strength?
Easiest way is if you bias the output tubes cooler, the trem will get more slam
-
Can you get the trem strength you want just by changing the bias pot?
See the 1meg on the plate of the osc. tube? Make that 470K or maybe even 220K to increase the trem signal.
-
See the 1meg on the plate of the osc. tube? Make that 470K or maybe even 220K to increase the trem signal.
Yeah, I was thinking of changing that. Thanks!
All the voltages are almost exactly the same as the original schematic including the bias voltage.
I can't remember if the PR's I've worked on had fairly weak tremolos.
-
yeah but not all output tubes are the same - so whether the bias voltage is the 'same' or not is not a primary consideration. You need to have the bias right for the actual tubes you have plugged in there. That could mean that you need to run them cooler to get more trem slam (with bias-wiggle trem). When the bias is cooler, it is easier for the trem voltage swing to put the bias into the cut-off region, otherwise it is fighting the tube current too much.
Besides using more voltage to drive the bias swing as Sluckey said, another thing you could do is drive the 6V6s with more current swing, but using a mosfet source follower after the LFO stage. I did this a little while back with a BFPR clone. (The circuit values I used for the mosfet were derived from the 5G9).
-
I've got the Princeton style trem on my Hoffman AB763 Deluxe Reverb using 5881's and it has plenty of strength- but I did have to fiddle with the bias to get the sweet spot, about 42 ma @450v on the plates. tubeswell is right, different tubes need different bias. I tried many tubes- several pairs of 6l6GC and 6V6 (both old and new) and the response was different for each set.
-
I tried many tubes- several pairs of 6l6GC and 6V6 (both old and new) and the response was different for each set.
Do you mean you had to re-bias each set -or - do you mean that after re- biasing each set of power tubes the trem sounded/worked differently?
Thanks, Brad :icon_biggrin:
-
It worked about the same for each set of tubes, I could always get it to work deeply by adjusting the bias. The different tubes made the overall sound of the amp different, especially when using the 6v6's compared to the 6l6GC and the 5188's. It's all about finding a balance point you like. I found the overall sound to be better with the 5188's, but others might like the 6v6's. The bias is much lower with the 6v6's, my transformers were from a 1978 DR, with a 5AR4 rather than a 5U4GB. That runs about 20 volts hotter than a PR or BF DR, it might be why I liked the sound of the 5881's.
-
Every tube biases a bit different. Set the bias as you would normally, listen to the trem and set to taste. Worked great. I have a NFB lift on this amp too. When lifted, the trem is stronger too.
-
Could you use a 1 meg pot and use it to vary the resistance instead of changinging to a 470K 0r 220K resister? Never seen it done but.....
-
IMO, can't have too much... I changed it to a 330k.
-
This post from the Archives covers the topic fairly well:
http://www.el34world.com/Forum/index.php?topic=604.0
I haven't tried the MOSFET buffer yet but plan to someday....
-
Neither one of my PRs has a "weak" tremolo. One is an original 78 Fender and the other is a Hoffman Board PR that folks here on this board developed. My HPR has mega surf trem, very adjustable from 1-10. I have JJs in it and I am running it a bit on the cool side. The Fender also has pretty hard core tremolo output. It's got EHs in it and biased not to hot and not too cold. I don't have the exact voltages for you, but I would say that tremolo is not a weak point in the PR.
my .02 cents