Thought I would share the results of my pseudo Wreck amp. The amp was based on the Kelly 90 print. The major differences were the transformers used. Power transformer was a Fender super reverb type and the ouput transformer was a Fender universal 18343 (4K p/p with 2, 4 and 8ohm taps). Still sounds very very good and I will be posting clips this week with 6V6's and EL34's.
Hey Gary,
Thanks for posting those voltages, I just received a big order from mouser and will be starting my build soon. Still got to get board materials and decide on iron.
Thanks for posting your voltages. I assembled mine as well and would be grateful if guys would check them out and let me know if any seem out of line. I had to add about 800k of resistance to my bias supply to get the bias voltage correct. I don't know if that is a routine thing to encounter with a build or not and I sure hope it won't hurt things.
BTW, this is a TR Ruby kit with their trannys.
614 Volts across PT HVs
6.8v across heaters (Why wouldn't this be 6.3?)
Biased at 70mv with El-34s.
B5 251
B4 264
B3 288
B2 387
B1 399
-5.7 on pin 5 of the power tubes doesn't sound like near enough bias voltage, to me. You say 70 mv, what you want to do is measure the current in miliamps. 70 ma seems reasonable for both tubes, but I would expect -30 to -38 or so at pin 5.
Here is a dialogue from a post I made earlier that will explain my bias circuit and how I am taking my readings. Following Gary's recommendation I add and additional 800K of resistance to my bias network and that allowed me to obtain a bias voltage as high as 76 on my bias point. That helped the amps performance greatly, but I think it could still be improved. I don't know if that much fiddling is normal. I followed the A1a schematic and the TR layout and values. Thank you for your help.
Robert
Posted: Fri Jul 08, 2005 2:43 am Post subject: Re: Biasing Question
I have just finished my Express build and am having trouble getting the expected bias readings I want. I have wired pins 1&8 together on each power tube, then I connect both wires together at an external bias point which is then connected to a 1 Ohm 5 watt resistor going to ground. Setting my DMM step to 200ma and establishing a ground to chassis with black probe, I get a reading of 16.6 on the bias point with the red probe. Changing the step to 20 ma, which I think is the wrong step, I get a reading of 3.35. All other voltages are in line:
B+1=397
B+2=387
B+3=287
B+4=263
B+5=250.
My Bias circuit is a 150K resistor off the secondary HV, a 1n4007 diode connected to a 100uF cap(with polarity correct) and a 25k pot followed by a 24k resistor to ground.
I'm sure I have done something stupid and would appreciate a quick bit of educating.
Thanks for all the help.
Robert
You need to set you DMM to voltage (MV) not MA. The resistor allows the voltage to be developed across it when current is flowing. 1mv / 1ma with a 1 ohm resistor.
I am guessing that you are probably shooting for something like 60 to 80 mV. Which will correspond to 60 to 80 ma on the two tubes combined.
Well, I use the transformer shunt method, myself, to measure current directly.
Is the resistor EXACTLY 1 ohm? Your measurement is reliant upon the accuracy of that resistor.
-5.7 volt bias voltage is far too hot for EL34's at that plate voltage. Forget the measurement of the current. Set the bias voltage to -38 and see how the amp sounds. Go hotter (decrease the negative voltage) to taste. I think -29 is as far as I'd go, personally. With the tubes I've used that's been 75% - 80% dissipation.
Here is a dialogue from a post I made earlier that will explain my bias circuit and how I am taking my readings. Following Gary's recommendation I add and additional 800K of resistance to my bias network and that allowed me to obtain a bias voltage as high as 76 on my bias point. That helped the amps performance
I'm still a bit in the dark about this comment, where did the 800K resistance go in the bias circuit and how did it improve the tone of the amp?
My bias circuit is(was) similar to the A1a schematic. From the PT Secondary wire, prior to the 3 diodes, I branch to a 150K resistor, then a 1N4007 diode, then connect to both 1)a 100uF 100v Cap to ground and 2) a bias pot followed by a 24K resistor to ground.
PT HV----->150K------>diode----->100uF Cap & Bias pot
With this arrangement I was getting 30-40mv at my bias point, which is created by combining Pins 1&8 on the power tubes and then uniting them at an external bias test point which is followed by a 1K resistor to ground. The tone of the amp was not that desireable, thus my post Express Observations.
In response, dobbhill wrote:
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Joined: 22 Jan 2005
Posts: 8
I believe your tubes are too cold, meaning the bias current is too low and your bias voltage (B-) is too high (too negative). With the cathodes of your power tubes connected together, you are looking to get 40mv + 40mv = 80mv total across the cathode (1ohm) resistor. You need to change the bias supply to give you less negative voltage by changing one or more of the resistors in the network.
Hope this helps, D
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After this, I increased the 150K resistor and added two more along my bias circuit until I reached 60-80 mv. This helped nearly every aspect of the tone, response and articulation of the amp. I really enjoyed it for a couple days, last night though it just didn't sound right, tone became grainy and thinner than I recalled. After Jackie's post, now I wonder if it is biased to high and the tubes are burning up.
Anyway, I am now confused and could use some help from you guys to sort the bias issue and the other voltages out.
Buschman wrote:Are you able to run 6v6s with your b+ in that 460 range without burning them up?
I have been using the JJ 6V6 tubes and they are pretty stout. As far as failures none to date. This amp does not see too much use but I have a lot of faith in the JJ's. I built an overdrive amp for a friend that has 450 volts of the plates of 6V6's and he gig's every week with the amp and uses it virtually everyday for a few hours. The amp has been going for 7 months now with no issues.
Gary
Located in the St Croix River Valley- Afton, MN
About 5 miles south of I-94
aka K0GWA, K0 Glas Werks Amplification www.glaswerks.com
