I recently finished fixing up a 1940s Tube PA for use as a guitar amp (see http://www.diyaudio.com/forums/tubes-valves/191232-looking-info-about-1940s-amp.html), and it's almost working perfectly.
The output is very muddy-sounding, with a pronounced treble rolloff. Testing with another amp, there's plenty of treble right up to the power tubes' grids. The output at the speaker has the treble rolloff. The amp uses two push-pull 6V6s with a Hammond 1639 OT (10k:8). All of the parts seem to be fine. I'm currently using two 6P6S's as power tubes. The schematic is here:
http://oi51.tinypic.com/vpkd2c.jpg
And these are the voltages:
A+ - 306V
Plate - 298V
Screen - 302V
Cathode - 21V
I've tested and tested and I still have no clue what's going on. Any ideas on where the treble rolloff is?
The output is very muddy-sounding, with a pronounced treble rolloff. Testing with another amp, there's plenty of treble right up to the power tubes' grids. The output at the speaker has the treble rolloff. The amp uses two push-pull 6V6s with a Hammond 1639 OT (10k:8). All of the parts seem to be fine. I'm currently using two 6P6S's as power tubes. The schematic is here:
http://oi51.tinypic.com/vpkd2c.jpg
And these are the voltages:
A+ - 306V
Plate - 298V
Screen - 302V
Cathode - 21V
I've tested and tested and I still have no clue what's going on. Any ideas on where the treble rolloff is?
Yes, I know that the problem is somewhere in the power amp, but I don't know exactly what it is. I'm wondering if anyone can help me pinpoint it or fix it.
They're almost unused. Just got them NOS, NIB, matched, and tested off of ebay, so I don't think that's the issue.
Wow, so that makes them great tubes, doesn't it? 🙂 You may be right but it's either that or the OT. Guessing here you already know your speaker doesn't sound muddy. Guessing also here that you tested the preamp output correctly.
I dont understand 'testing with another amp, and u got treble to the grids'? Did you drive the output tubes with another amp? Did you mean you compared this amp with another amp?
Anyways, looking at the schem it seems to me the amp is still designed for PA and not guitar. Guitar amps need to be tuned for a brighter response and not like a HiFi amp with flat response. Make it so bright it hurts, and I bet you'll get a better tone for guitar.
I took the output from the preamp at the power tubes' grids, and plugged that into the input of a different amp. It sounded normal and there was no treble rolloff. When I took the output from a line out box connected to the speaker, there was a significant treble rolloff. So, I'm losing treble somewhere in the power amp. Also, I'm sure that it isn't a flat response. I could tell if it was. There is a significant treble loss in the power amp, and I'd don't want to start messing around with the tone until I find and fix it.
@Cassiel
Of course I'm not 100% sure that the tubes are okay, but I don't own a tube tester or any other new 6V6s so that's as close as I can come to a guarantee. It could be anything, but I'm more likely to suspect the OT than the tubes. Any ideas about how I can pinpoint or fix the treble rolloff?
@Cassiel
Of course I'm not 100% sure that the tubes are okay, but I don't own a tube tester or any other new 6V6s so that's as close as I can come to a guarantee. It could be anything, but I'm more likely to suspect the OT than the tubes. Any ideas about how I can pinpoint or fix the treble rolloff?
Treble loss or bass peak? With a relatively high output impedance you will get a peak at the drive unit bass resonance. This may sound like a treble rolloff.
What test equipment do you have (or have access to..) ?
Sine/square wave generator?
Oscilloscope?
VTVM or good meter?
@DF96
I get the problem running through 8 ohm speakers, but I suppose it could be an OT resonance thing. It sounds distinctly like a treble rolloff, not a bass peak, but that could be due to the tubes compressing and the bass dominating the signal. Do you know any sites with good information about OT frequency responses and resonances?
@VictoriaGuy
I have an oscilloscope and a generic multimeter. I can use my laptop as a sine/square/noise generator and as a frequency counter, spectrum analyser, oscilloscope, etc. I can't do any real testing until this weekend, but I plan to do some frequency response tests. What else do you suggest?
I get the problem running through 8 ohm speakers, but I suppose it could be an OT resonance thing. It sounds distinctly like a treble rolloff, not a bass peak, but that could be due to the tubes compressing and the bass dominating the signal. Do you know any sites with good information about OT frequency responses and resonances?
@VictoriaGuy
I have an oscilloscope and a generic multimeter. I can use my laptop as a sine/square/noise generator and as a frequency counter, spectrum analyser, oscilloscope, etc. I can't do any real testing until this weekend, but I plan to do some frequency response tests. What else do you suggest?
That's what I'd suggest.
🙂
Running into a dummy load might also give some ideas vs. using the speakers.
Can the computer output a square wave and monitor the response at another spot in the amp using the scope function (simultaneously)? It sounds pretty useful.
OK, I'll work on that this weekend.
It can, although I find it more useful to use white noise as a source for frequency analysis and sine waves for distortion analysis. I just use a VST synth for the waves and then Voxengo SPAN or Visual Analyser for analysis.
It can, although I find it more useful to use white noise as a source for frequency analysis and sine waves for distortion analysis. I just use a VST synth for the waves and then Voxengo SPAN or Visual Analyser for analysis.
Well, you are way ahead of me!😀
I'm still playing with some old Heathkit stuff and a cheap scope here.
I'll be watching here; not sure I have much to contribute.
Good luck with the troubleshooting!
I'm fairly new to tube amps but I've found that it's best with these things to check out the most obvious potential sources of trouble first. So I was wondering if maybe it has something to do with the output transformer. Does it have multiple output taps? If so are you 100% certain that it is hooked up correctly? If it has 16/8/4 ohm taps, the 8 ohm would be in the middle and you might be hooking up the wrong half of the secondary (i.e. 8 and 16 instead of 8 and common). I would doubt that a properly-working output transformer would cause a treble rolloff, more likely a bass rolloff if it is undersized as transformers tend to be pretty efficient at higher frequencies.
I did a bit of testing, and I couldn't find the rolloff without tearing the whole thing apart. I just decided to throw on a bright cap and be done with it. It works fine now and sounds great. Loads of great distortion, although it's pretty quiet. I'm interested in a new speaker but there are only six speaker screws instead of eight.
Anyway, I'm curious about what's wrong, but I fixed it enough and I'm happy.
Anyway, I'm curious about what's wrong, but I fixed it enough and I'm happy.
- Status
- Not open for further replies.