Search found 384 matches

by dave g
Fri Apr 25, 2014 8:46 pm
Forum: Trainwreck Discussion
Topic: Express Output Waveform Shots
Replies: 205
Views: 40817

Re: Express Output Waveform Shots

Merlin Blencowe has some discussion on overdriven LTP duty cycle modulation in both editions of his Preamp design book. In the second edition there is an illustration of how the selected bias point determines which side will increase in width. The Express' LTP is biased hot, which according to Blen...
by dave g
Sat Apr 19, 2014 2:04 pm
Forum: Trainwreck Discussion
Topic: Express Output Waveform Shots
Replies: 205
Views: 40817

Re: Express Output Waveform Shots

I don't get how the 3rd stage could be changing its duty cycle input to output when there's no cap or anything to shift bias. The duty cycle shift results from the fact that when it clips only one side of the input wave, it changes the average value of the wave. Take the integral of the clipped sin...
by dave g
Fri Apr 18, 2014 1:48 am
Forum: Trainwreck Discussion
Topic: Express Output Waveform Shots
Replies: 205
Views: 40817

Re: Express Output Waveform Shots

katopan wrote:We need tests, not conjecture.
Yes, I think that was my suggestion.
by dave g
Thu Apr 17, 2014 1:05 pm
Forum: Trainwreck Discussion
Topic: Express Output Waveform Shots
Replies: 205
Views: 40817

Re: Express Output Waveform Shots

If anyone has a SPL meter they should measure the actual loudness of their amp when rolled back for clean vs. distorted. It would not surprise me if the distorted sound is about 3 dB louder than the clean sound.
by dave g
Thu Mar 20, 2014 3:23 am
Forum: Trainwreck Discussion
Topic: Rating of Ohmite Resistors in Concorde/K60?
Replies: 2
Views: 599

Re: Rating of Ohmite Resistors in Concorde/K60?

If you assume 50 mA per tube...

Power = Voltage * Current

Ohm's Law states that V = I*R, and in this case R = 1, so we've got that V = I, so power = I^2 = 0.05*0.05 = 0.0025 watts.

A 1/4 watt resistor is capable of dissipating 100 times more power than you need, so don't worry too much about it :D
by dave g
Sun Mar 16, 2014 4:51 pm
Forum: Trainwreck Discussion
Topic: 2x EL84 Liverpool
Replies: 14
Views: 3462

Re: 2x EL84 Liverpool

I guess I'd go with the 1650F, but the 1609 would probably be okay. Primary impedance isn't really a big deal as long as you're in the ballpark. Speakers are wildly reactive loads whose impedance varies quite a bit with frequency, so your output tubes aren't going to see a nice, constant 10k load re...
by dave g
Sat Mar 08, 2014 6:16 pm
Forum: Technical Discussion
Topic: Have you tried buffering power valve grids?
Replies: 4
Views: 893

Re: Have you tried buffering power valve grids?

The zeners may not do their job when you turn down the VVR since their reverse breakdown voltage is fixed, yet at lower VVR settings you will need less swing to drive the output tubes into distortion. I haven't tried it though and it's entirely possible that they would still work. I think a better s...
by dave g
Fri Feb 07, 2014 2:48 pm
Forum: Marshall Discussion
Topic: 18 watt PI problem
Replies: 3
Views: 965

Re: 18 watt PI problem

Have you tried a different tube?
by dave g
Fri Jan 17, 2014 12:56 am
Forum: Technical Discussion
Topic: New pickup technology?
Replies: 27
Views: 3265

Re: New pickup technology?

7. You want a really cool "different" line of pick-ups? Check the Zexcoil. I am not affiliated, just like the concept and execution. Humbucking across coils for each string? Does that mean I'll have a dead spot if I try to play a unison across the wrong pair of strings? Sounds complicated and kind ...
by dave g
Fri Jan 10, 2014 8:15 pm
Forum: Technical Discussion
Topic: Construction Practices
Replies: 40
Views: 6485

Re: An education

So after reading up on grid clamping.....a term unknown in engineering of my day since it would operate a tube outside of its specifications.... I am asking why drive a control grid so hard it draws appreciable current. It has a hard enough life being so close to the cathode and its wires warp so e...
by dave g
Thu Jan 09, 2014 9:02 pm
Forum: Dumble Discussion
Topic: nasal tone
Replies: 4
Views: 1174

Re: nasal tone

Is this just on the OD channel? Try replacing the OD tube.
by dave g
Sat Jan 04, 2014 8:42 pm
Forum: Technical Discussion
Topic: Construction Practices
Replies: 40
Views: 6485

Re: Construction Practices

I am also wondering why there is very little use of drivers between the phase inverter and output pairs or quads in higher power versions using 6550s or similar. Most guitar amps don't use drivers because grid clamping on the output tubes is desirable. People typically call it "power tube distortio...
by dave g
Fri Dec 13, 2013 6:06 pm
Forum: Dumble Discussion
Topic: chewy notes on D-style amps
Replies: 101
Views: 11969

Re: chewy notes on D-style amps

briane wrote: if you aint using the volume knob on the guitar, then you aint using a dumble correctly....IMO I seem to recall dogears (Scott L) rarely rolled his volume back from 10 :wink: I think the best way to use a Dumble is to do whatever you like... IMO Paul. Agreed. Guitar tone controls real...
by dave g
Thu Dec 12, 2013 8:06 pm
Forum: Technical Discussion
Topic: Corrective filter in 4xEL84 amp
Replies: 34
Views: 6148

Re: Corrective filter in 4xEL84 amp

Unrelated to conjunctive filters, but relevant to the EL84 fizzyness: Most EL84 amps use a bias voltage around -10 volts. Contrast this with 6L6s, EL34s, etc. which require on the order of -30 or -40 volts (in the typical fender/marshall push-pull output circuit). EL84s require far less signal from ...
by dave g
Mon Dec 09, 2013 8:34 pm
Forum: Dumble Discussion
Topic: chewy notes on D-style amps
Replies: 101
Views: 11969

Re: chewy notes on D-style amps

Typical 12" guitar speakers are very nonlinear (in a bad sounding way) at extremely low power levels. IMO you need to be putting at least a watt through them before they start sounding good. IME, harsh OD on D-style amplifiers that you can't seem to get rid of by dumping treble is often due to V2b b...