First Build - Marshall 18 Watt build troubles

[bandade]

Hi I am new to the forum and amp building; This is my first build. I have been putting togther a Brown note amp similar to a Marshall 18. Here is the schematic:
http://www.brown-note.com/schematics...HEMATIC_2B.jpg
I used a Classic Tone Transformer rather than the Brown note one and have 3 individual output jacks rather than a selector switch. Capacitors are SOZO, Sprague, and Mallory. All the voltages on the tube pins are correct when compared to other peoples voltages.

My problems are:
1) I have a consistant hum when "Standby" is turned off. The volume knob does amplify the hum when it is turned up.
2) With low volume and tone control completely turned down, I can get a nice clean sound. When tone control is turned up past 8:00 There is a loud consistant squeel which changes frequencies as the tone knob is rotated.
3) When volume is pushed to overdrive, the sound is fizzy and lacks sustain.

Can anyone give me insite into what these problems may be caused by? I believe they are all related and would probably be a pretty easy fix if i could identify the fault. I don't have a tube checker but given that the pin voltages are correct and there is no abnormal glowing, I think they are still good. I also tapped lightly on each tube when the amp was on to listen for anything to indicate a microphonic tube and heard nothing out of the ordinary. I will keep probing around.

Thanks

[Wombaticus]

Hi I am new to the forum and amp building; This is my first build. I have been putting togther a Brown note amp similar to a Marshall 18. Here is the schematic:
http://www.brown-note.com/schematics...HEMATIC_2B.jpg
I used a Classic Tone Transformer rather than the Brown note one and have 3 individual output jacks rather than a selector switch. Capacitors are SOZO, Sprague, and Mallory. All the voltages on the tube pins are correct when compared to other peoples voltages.

My problems are:
1) I have a consistant hum when "Standby" is turned off. The volume knob does amplify the hum when it is turned up.
2) With low volume and tone control completely turned down, I can get a nice clean sound. When tone control is turned up past 8:00 There is a loud consistant squeel which changes frequencies as the tone knob is rotated.
3) When volume is pushed to overdrive, the sound is fizzy and lacks sustain.

Can anyone give me insite into what these problems may be caused by? I believe they are all related and would probably be a pretty easy fix if i could identify the fault. I don't have a tube checker but given that the pin voltages are correct and there is no abnormal glowing, I think they are still good. I also tapped lightly on each tube when the amp was on to listen for anything to indicate a microphonic tube and heard nothing out of the ordinary. I will keep probing around.

Thanks

Sounds like lead dress issues to me. In cases like this, some hi-res "gutshot" photographs can be very helpful indeed.

[pontiacpete]

you could try reversing the brown and blue wire on the output transformer. It could be oscillating.

good luck.

pete

[Twist]

I know Fender used to do this and you may have too, but don't solder your ground buss across the back of the pots. Instead isolate the ground buss starting at the input jack all the way back to a star ground at the chassis. Your speaker transformer ground and speaker ground can be grounded at the speaker jack. Ground your filter caps at the star ground. Attach your AC ground and filament ground to the chassis but away from the star ground.
Make sure you have a filament center tap ground. The oscillation probably is a lead dress issue like the other reply stated.
When I first started building tube amps I too had many strange hum problems from amp to amp. The grounding scheme and layout is the most critical part.

[bandade]

Thanks for the insite thus far everyone. I did some more probing around yesterday and did make some progress. I switched the blue and brown wires from the output transformer and it removed the oscillation. I am still getting some oscillation when I have the amp cranked and I go to switch pickups on my guitar.

I have also made progress with the hum issue. There is still a small amount of hum but even with the amp cranked, it is only a small amount which i would expect. It may be worth noting that the hum changes frequencies when i tweak with the volume and tone knobs on my guitar. Yes, i was using a Strat but it was in the second position which is hum canceling. I should also say the hum is consistant whether I am using 1 single coil pickup or using 2 with opposite polarity. As I said before, I used a Classic Tone transformer (http://www.classictone.net/40-18035.pdf) I had origionally connected the 6.3V White and 0V Yellow leads to the EZ81 for the heater as suggested in the schematic but was reading 120 - 140V AC on the rectifier heater pins so i disconnected them and ran leads from the heater terminal used for other the other tubes so that I could get a normal 3V on the EZ81 heater pins. Last night I connected the Yellow and White wires of the power transformer to the heater terminals and am still reading 3V with far less hum. The yellow/white wire is at its own terminal and is not being used.

One problem I need to work on still is that when the amp is pushed into overdrive. The overdrive is fizzy and has very little sustain. It sounds like I have a bad guitar cable which is not the case as I have tried many. I think it may be the gage wire I used. I used wire rated to 300V and I now know that there are several connections that have around 350V running through them. I will change to higher rated wire on those high voltage connections tonight and will recheck for lead dress issues again. Last night I used a wood scewer and moved wires around trying to find any lead dress issues and found only one that was slightly loose.

Thanks,
Dan

[frankeg]

Dan, I agree with what has been offered so far. Swapping the OT wires helped but it sounds like you still have some ocillating going on. Some things you can try: 1. Isolate by pulling one pre-amp tube at a time noting what changes. 2. Low buzz can be caused by a bad power supply cap, check them (even new ones can be bad sometimes). Good luck.

[frankeg]

I had origionally connected the 6.3V White and 0V Yellow leads to the EZ81 for the heater as suggested in the schematic but was reading 120 - 140V AC on the rectifier heater pins so i disconnected them and ran leads from the heater terminal used for other the other tubes so that I could get a normal 3V on the EZ81 heater pins. Last night I connected the Yellow and White wires of the power transformer to the heater terminals and am still reading 3V with far less hum. The yellow/white wire is at its own terminal and is not being used.

Dan, I don't understand this... Your EZ-81 should have 6.3 vac heaters which based on your transformer should be the Yellow wire and white wire. The Yel/Wht wire is the 5 volt tap for a different style rectifier. You said you originally ran the white and yellow wires but got 120-140 vac but it looks like you ended up with the same two wires with 3v ? I would still check your power caps. Also make sure the polarity is correct on all electrolytics (an easy mistake to make)....frank

[defaced]

1) I have a consistant hum when "Standby" is turned off. The volume knob does amplify the hum when it is turned up.

What does it do with the power tubes pulled and the stand by off and then again on? I'm thinking a chassis layout issue on this one.

[bandade]

Thanks for the help everyone. Almost everything you guys have suggested has helped. Its been a while since I last gave an update but this is where I am. I did have a few questionable lead dresses that I fixed. The main cause of the hum was a chassis layout issue. Amerature mistake i suppose but my power transformer and other high voltage components where to close to my input jacks and volume/tone pots. I rearranged where the low voltage components were located relative to the high voltage and it corrected the hum problem. The only Hum that I have remaining is from the LOW input jack and I think it is likely a grounding issue which I can figure out as soon as I really take another look at it.

The amp sounds absolutly killer in the HIGH input! When I crank the amp and strum and play i get a nice overdrive but when I let the notes ring out until they transition back to clean from overdrive, the transition isn't all that smooth. I've read that SOZO capacitors take around 100 hours to break in so I have spent around 15 hours burning them in and the transition from overdrive to clean has improved somewhat, I will continue the burn in and hopefully the transition will smooth out even more.

I have been focusing most of my time on building the speaker and head cabinets which I will be covering with tolex this weekend. I decided to go with a 212 Dumble/Two Rock style speaker cabinet and a Plexi Style head cabinet; light blonde tolex, black grille cloth, white piping, black faceplate, and cream chicken-head knobs.

Thanks again for the help.

[bandade]

I spent some time trying to resolve the hum on the low input last night and found out it was just simply that 1 of the contacts wasn't grounding on the input not in use. I bent the contact with some pliers and problem solved. No more hum when using either input or when no guitar is plugged in at all. The amp sounds great! I'm glad I fixed the 2nd input because I actually prefer the sound when plugged into it.