Help troubleshooting 50w build.

[waltermatthew]

Hey all.

Finished assembling my 50 Watt/Metro/Valvestorm/Classictone tranny build and need a little help troubleshooting.

First off, it plays and sounds good minus the following issues.

I'm having trouble with the biasing. I'm getting a reading of 20mv across the 1 ohm resistors with the bias pot all the way up. As I turn it down it will fall all the way off to 0. My HT reading is actually 478ish volts. Any ideas? I checked the bias resistors on the board and they are correct...and the orientation of the Caps is correct as well.

Addiontally; I'm getting a pretty nasty hiss when I plug into the bright channel...It's not there on the normal channel(significantly quieter). I used Larry Grounding and was hoping that would take it all away...but I also implemented a LarMar PPIMV and followed the instructions by not grounding the wire shield...Should I have done something with this? I put the pot in a speaker hole...any suggestions on wiring?

Thanks for your help

Matt

[waltermatthew]

Also...I used brass washers to pull between the grounding points and the chassis....would that have any effect as opposed to to lock washers?

[ivan H]

Hi, the idea with using the star shaped lock washers between the grounding lugs & chassis is so they bite into both the chassis & ground lug to ensure a good reliable connection, which a brass washer wont do. Thats not to say u dont have good connections at the moment tho it might be an idea to change them. Be carefull with the brass bolts, its easy to over tighten them, especially the 2nd time round. The bright channel is suceptable to hiss, keep the plate lead against the chassis & the grid lead up away from it & other leads. Also a quiet tube in the first position. The shield on the leads to the PPIMV can be connected to the negative bias voltage so as to shield the inner wire, if u've done this, thats the reason it cant be grounded. If u havent, connect it up, the bias voltage will act as a shield just as effectively as grounding it will (or alternatively, ground it, but only if it IS'NT connected to the bias voltage). Hope this is of some help. Cheers

[neikeel]

Hiss is signal coming through at idle, usually if you have a big bypass cap early in the signal chain, good component choice helps to an extent. Higher gain amps are prone to it. Hum is something that the grounding will help with and unusual squeaks and squeals of PO will be avoidable with neat lead dress.

[waltermatthew]

Neil-It's only the electron sounding hiss that I'm getting it's just loud and I was trying to remember if my old long gone marshall had a similar sound. Hardly any hum there.

Anybody have an idea why I'm measuring only 20mv across the bias point? I'm getting -40.6v on the opposite pins.

[waltermatthew]

Okay..clipped the bright cap and no hiss...dead quiet...now if I could figure out the biasing I'm set....What's the purpose of the diode in the bias circuit?

[waltermatthew]

Should I drop the resistor feeding the 25k bias pot as they surmised in this post?

http://forum.metroamp.com/viewtopic.php?f=4&t=37684" onclick="window.open(this.href);return false;

[frusciante89]

I'm having the same problem with the hiss.. what do you mean with "I clipped the bright cap"?

[demonufo]

Got rid of the bright cap on the volume pot. This will make an appreciable difference at lower volume levels, but personally I feel these amps need at least SOME kind of treble bypass cap, otherwise they suffer greatly at lower levels, especially for those who like to vary the volume control on the guitar itself.

[danman]

I had the same biasing issue with my 50 watt build. Every set of tubes that you may use will draw different amounts of current at idle and this may be outside of the range allowed by your bias pot and resistor combination. If you used the 47k resistor inline with your bias pot you will need to go down a bit to get your idle current where it needs to be. I believe I had to use a 33k instead to get the bias set properly. This is not uncommon as every set of tubes is slightly different. I also noticed either more or less hiss in my bright channel depending on which bright cap I tried. I believe I settled on 100pf there and have been pretty happy since.