I agree,
This circuit is only a pre-amp stage. There is something wrong with the PSU to have that kind of AC on top of the B+.
A 47uF+47uF with a resistor should perform well enough.
The Mains Tx isn't short primary -secondary is it?
Arnulf, you have got to find this one! We won't sleep if you don't
Regards
M. Gregg
All guitar amp circuits I've seen have a volume pot after the 1st stage. Using one in your setup would reduce noise (when the volume's not maxed) and allow for optional preamp distortion capability ("good") when used in conjunction with lower settings on the existing volume control. Since DC heaters in the preamp did not resolve the hum I'd be also looking for a ground loop and/or other heater wires laying parallel to preamp signal wires. The AC figures in the PS are perplexing unless your meter is low quality or some filters are not filtering.
Direct. I'm going to give your suggestion a try, I'm desperate as it is
@pedroskova: I'm not putting as much faith into that reading anymore - I disconnected everything except OPT B+ connection (it has no ground return when output tubes aren't conducting) and the meter still reads 15V AC on top of 260+V DC at either of first three filter capacitors (which are on the same board). It just doesn't make any sense. I'm going to try a different rectifier, two capacitors and one resistor, to see whether it makes any difference. If it does I'm going to use that as separate anode supply just for the two sections at the input.
@trobbins: I used the regular "HiFi" amplifier parts I had on hand (I put everything together on New Year's eve) so capacitors are too large for guitar etc. I'm going to take care of that, I just haven't made it to a store where they sell electronic bits yet
As for this guitar socket, it has an integrated switch so I can make it short out when nothing is plugged in. FWIW, with input shorted (= guitar volume potentiometer turned all the way down) the hum is much more bearable, so cable is to blame for significant portion of noise.
While on the subject of cable picking up noise, would differential input stage with high CMRR help alleviate this problem or would it be a waste of tube (I'd rather use mu-follower with its low Zout and high PSRR instead), considering that guitar cable has only one internal conductor and shield around it so I guess it can't pick up common mode noise ? I don't think I've ever seen LTP used at the input of guitar amplifier, I suppose there's a good reason for that
@jjman: I'm going to rebuild everything differently. I'm going to physically remove both sections of the input (= one tube) from this board and move them into the vicinity of both guitar socket and potentiometers, which means extremely short wires (instead of having to run them back and forth from front panel to back of the enclosure). I might also enclose the input tube in a small metal box of its own (amplifier enclosure is made from MDF material, the only metallic bit is a small front plate above the speakers where switches, knobs, socket and indicator are going to end up mounted, and another board to hold the transformers and circuit board).
I was also contemplating use of sandy devices for regulation but I'll only resort to that if I can't get rid of hum any other way. In this case I'm going to give DC heater one more chance, but this time with regulated or better yet CCS supply (I think I can squeeze it in if I use schottky rectifiers).
Output stage appears to be fine, it plays very nicely when I connect my portable mp3 player to it. At least something works as it is supposed to
Typ guitar input is high gain (to keep costs down), so standard resistive coupled amp with ac bypassed cathode is de rigeur. Given use of suitable valve designed for low noise (eg. 12AX7), and suitable wiring/powering etc, and cabling, then as most appreciate it achieves pretty low hum - so if you are experiencing not so low hum, then best to look further into issues than to jump from frying pan into fire.
Okey dokey, I managed to borrow a decent DSO (scope) and here's what I've determined:
1: Output section (one common cathode stage, AC coupled cathodyne and PP output tubes) produces minimal 100 Hz noise with volume potentiometer (located before common cathode stage) all the way down. I was wrong to blame the PSU for the noise problems.
2: IT is 50 Hz hum that has been bugging me and it's apparently picked up by cabling. I replacted both input cable and potentiometer cable and hum has gone down slightly. As stated in my previous message I'm going to redesign this thing in order to keep cable length to an absolute minimum.
3: When I touch amplifier's ground or guitar strings, 50 Hz hum goes down by 30-50%. This explains why PSU noise injection (100 Hz) didn't do a darn thing, what I need to do is to inject 50 Hz instead, if I decide to follow this path.
Situation sure is looking brighter today
1: Output section (one common cathode stage, AC coupled cathodyne and PP output tubes) produces minimal 100 Hz noise with volume potentiometer (located before common cathode stage) all the way down. I was wrong to blame the PSU for the noise problems.
2: IT is 50 Hz hum that has been bugging me and it's apparently picked up by cabling. I replacted both input cable and potentiometer cable and hum has gone down slightly. As stated in my previous message I'm going to redesign this thing in order to keep cable length to an absolute minimum.
3: When I touch amplifier's ground or guitar strings, 50 Hz hum goes down by 30-50%. This explains why PSU noise injection (100 Hz) didn't do a darn thing, what I need to do is to inject 50 Hz instead, if I decide to follow this path.
Situation sure is looking brighter today
is it possible to use humdinger (say 2X100ohms fixed to GND) on the final stage ie 6550 heaters? what about DC elevation instead of fixed humdinger for final stage?
Whoa, back one step. Please describe your heater supply setup for your amp? Is it one heater winding for all valves - as it sounds like you have two transformer windings?
From 2x 25R, to 2x 100R is typical of a fixed humdinger - with centre point connected to star 0V. This can be done for each heater winding. You can then temporarily clip lead a pot (~1k) across one side of the humdinger and see if hum tunes better, and then swap to other side of humdinger. If you get a much better result with some level of the pot, then measure the pot and replace with a soldered in fixed resistor.
Ciao, Tim
From 2x 25R, to 2x 100R is typical of a fixed humdinger - with centre point connected to star 0V. This can be done for each heater winding. You can then temporarily clip lead a pot (~1k) across one side of the humdinger and see if hum tunes better, and then swap to other side of humdinger. If you get a much better result with some level of the pot, then measure the pot and replace with a soldered in fixed resistor.
Ciao, Tim
I used to believe in grid stoppers in kiloohms or tens of kiloohms range and gave this one a try only after one of more recent threads here where a forum member has argued that too large a grid stopper may have no effect at HF. IIRC his statement was to keep grid stopper value well below 1K and this is the first amplifier where I went with his suggestion.
I do believe the floating paraphase PI was responsible for some of problems though as situation changed drastically when I reverted to straightforward cathodyne. All the NFB involved is internal to cathodyne, rather than routed through 1M resistors and around the circuit board, which certainly helps alleviate any potential oscillations. The meter I'm testing with is a 1 Gsps 100 MHz scope and the only thing that stands out when using it is 100 KHz peak in background noise that is roughly equal to 50 Hz hum. I'm confident the combination of grid-cathode capacitance and existing stoppers is taking care of it though, plus this may be just an anomalous reading which results from using the scope (it appears to have an internal SPMS which works off of internal battery).
@pedroskova: I'm afraid there is no such thing as "cheap Edcor transformer" for Europeans. It's cheaper to have transformers custom wound per user's specification, plus things can be overbuilt (in the case of OPTs and mains transformers) for minimal additional charge, literally an equivalent of one or two beers at the pub
You need larger core ? No problem, it's few per cent extra. Need thicker wire ? Few per cent extra. There's no 50-100% additional cost when going up in power by one rank.
I've recently built three guitar amps. First was derived from the famous Ken Fischer Trainwreck amp known as a Liverpool. Second was a 5F1 Champ derived amp and third was another Wreck similar to the first.
All three used 6N2P and 6P1P tubes, and the Wrecks used a 6J9P for the input first stage. The wrecks are High Gain lead amps designed for overdrive distortion. The Champ like amp is a little 4W practice amp.
I've seen similar hum problems with all three amps. The Champ was worst because the on lamp I used had one side to ground. When I disconnected it and used two 220R resistors from the heater to gnd it reduced the hum considerably.
Input to the first tube needs to be coax as this is the most sensitive wire in the amp. I used RG174U.
From Volume pot to next tube was the second most sensitive wire I found and using coax there helps as well.
Beyond that lead dress seems to be a big factor along with ground structure.
In "Designing Valve Preamps", Mr. Blencowe recommends either e a ground bus or star ground as using the chassis as a ground tends to result in more hum.
I run twisted pair for B+ and ground to the first two gain stages and found that helped as well.
Finally, I've been told by several guitar players that touching the strings normally reduces hum. I've seen this in all three of my amps and it does not make sense to me. My body is floating wrt the ground and should act as an antenna making hum worse. If I unplug the guitar and touch the chassis or input jack (floating with it's own ground wire), hum gets worse confirming I act like an antenna in this case. Orienting my body when holding the guitar also effects hum to a great extent. Moving away from the amp resuces it.
Florescent lights S*&k! Turn them all off.
Good luck.
All three used 6N2P and 6P1P tubes, and the Wrecks used a 6J9P for the input first stage. The wrecks are High Gain lead amps designed for overdrive distortion. The Champ like amp is a little 4W practice amp.
I've seen similar hum problems with all three amps. The Champ was worst because the on lamp I used had one side to ground. When I disconnected it and used two 220R resistors from the heater to gnd it reduced the hum considerably.
Input to the first tube needs to be coax as this is the most sensitive wire in the amp. I used RG174U.
From Volume pot to next tube was the second most sensitive wire I found and using coax there helps as well.
Beyond that lead dress seems to be a big factor along with ground structure.
In "Designing Valve Preamps", Mr. Blencowe recommends either e a ground bus or star ground as using the chassis as a ground tends to result in more hum.
I run twisted pair for B+ and ground to the first two gain stages and found that helped as well.
Finally, I've been told by several guitar players that touching the strings normally reduces hum. I've seen this in all three of my amps and it does not make sense to me. My body is floating wrt the ground and should act as an antenna making hum worse. If I unplug the guitar and touch the chassis or input jack (floating with it's own ground wire), hum gets worse confirming I act like an antenna in this case. Orienting my body when holding the guitar also effects hum to a great extent. Moving away from the amp resuces it.
Florescent lights S*&k! Turn them all off.
Good luck.
TheGimp, we're seeing the same thing with regards to touching the strings: hum goes down although it doesn't show on oscilloscope (just trucklaod of what looks like random white noise). When I touch the tip of the cable (with plugged into socket - it's one of those sockets which isn't completely enclosed in plastic) hum gets REALLY LOUD.
I hope to be able to rebuild everything over the course of the coming week. I'll definitely keep you guys updated with my adventures into guitar amplifier world
I hope to be able to rebuild everything over the course of the coming week. I'll definitely keep you guys updated with my adventures into guitar amplifier world
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