convert to HRM

[dcribbs1412]

Hi guys
got a weird oscillation in a build I converted to HRM(dlite)
playing soft with very low volume its OK.When I dig in it sputters and crackles. Speaker and cables are good
Voltages are V1 190's V2 low 200's V3 low 300's
Changed V1 caps, Vol pot, a few shielded cables.Went back and checked grounds, reflowed connections.Also tried different tubes.
Running el34's with a 50 watt MC Marshall OT.PT is the stock dlite.
gotta be something I'm overlooking...btw I think the OD relay is working backwards. Any suggestions are very much appreciated

Darin
sorry about the crappy pics
http://i1018.photobucket.com/albums/af3 ... attHRM.jpg
http://i1018.photobucket.com/albums/af3 ... HRM009.jpg
http://i1018.photobucket.com/albums/af3 ... HRM007.jpg
http://i1018.photobucket.com/albums/af3 ... HRM010.jpg

[CHIP]

Darin, This may be unrelated to your problem,but do you have send and return jacks on this D'lite?
I had what your describing happen to me. It was the switching clip on one of those newer Switchcraft jacks. I used a piece on 2000 grit sand paper to get in between the contacts. Corrected my problem.
Of course this situation only occured when nothing was plugged into the effects loop.

[dcribbs1412]

Thanks CHIP
tried cleaning the jacks and plugged in a dlator...
sputtering and crackles still happening.

Darin

[dcribbs1412]

Quick question
How can I use the fx loop to test if the preamp is the cause or the power amp...using another D style amp.
any help is appreciated

Darin

[tubedogsmith]

Does it make the noise without you playing through it? If so, pull the first preamp tube and if it stops its in that section. If not pull the nest two, one at a time. If it's still makeing noise it's most likely in the power section.

This isn't entirely foolproof but it saves a lot of time and works 98% of the time.

[alvarezh]

Quick question
How can I use the fx loop to test if the preamp is the cause or the power amp...using another D style amp.
any help is appreciated

Darin

Hi Darin, just go from preamp out (send) of the HRM amp into the poweramp in (return) of the other amp, that checks the preamp. To check the power section, go from preamp out of the other amp into the poweramp in of the HRM amp.

Do check if your power tubes are red plating (while oscillating), some sputter, crack and hish strongly (like a super gainy amp) when they do. This happens because the oscillation is retrofeeding and causing too much signal into the power tubes. The oscillation could be caused by bad lead dress.

All the best.

[dcribbs1412]

Does it make the noise without you playing through it? If so, pull the first preamp tube and if it stops its in that section. If not pull the nest two, one at a time. If it's still makeing noise it's most likely in the power section.

This isn't entirely foolproof but it saves a lot of time and works 98% of the time.

Thanks tubedogsmith
noise only happens when i dig into a note...slight hum when its running
nothing big

Hi alvarezh
no noticeable red-plating...will redo some leads
going to try and isolate problem to pre or power section
I want to learn what is going on for the future, just sorta frustrating
to make a few changes to an amp that was OK and now its really bad.
I'm hoping it will be worth it if I get it going. I have not had good luck with HRM's...so far.

Thanks
Darin
Update
just tried using another amp as power and no problems
actually sounds decent (cleans)
I'm guessing its in the power section.looking at replacing resistors
and redoing leads around the power tubes
Any suggestions are greatly appreciated

Thanks
Darin

[Bob-I]

What's the bias current? Biased too low can cause extreme crossover distortion and can sputter and crackle.

[dcribbs1412]

What's the bias current? Biased too low can cause extreme crossover distortion and can sputter and crackle.

Thanks Bob
iircc around -55...I will double check ASAP
Should voltage be around -40? I'm running EL34's
I have a 22k on bias pot and using 200k's for bias feed.

Darin

[pedro]

Hi,
you may know this but.....
If you have 1ohm resistors fitted from cathode to ground you simply measure the voltage in mV across this resistor to see what reading you get.

as the resistor is 1ohm ( and should be 1% tolerance ideally ) then the reading in mV is the equivalent current in milliamps. Set bias accordingly

http://www.webervst.com/tubes1/calcbias.htm

the weber bias calc is good reference point - adjust your bias current based on your plate voltage reading

Rgds
PeteC

[dcribbs1412]

Thanks pedro
I do have the bias resistors in place
biased at around 30ma bias voltage is -41v
replaced bias pot and bias feed resistors...
all voltages seem good
will continue to replace filter caps one at a time.
...not sure whats going on

Darin

[pedro]

whats your plate voltage measure ?
Rgds
PeteC

[dcribbs1412]

whats your plate voltage measure ?
Rgds
PeteC

450V....biased at 31-33

Thanks

[pedro]

bit on the low side for el34 in class AB - but running low is reported as good in many cases here YMMV

must be something else.

have you tried the PS caps - bad caps will lead to ghosting and "barking" sometimes.


Rgds
PeteC

[dcribbs1412]

Hi Pete
I have replaced the 2 100uf caps( with a 100uf 450v and a 220uf 350v)not sure what effect this will have but it's
what I had on hand... and 2 of the 4 22uf
will be replacing the others asap. will report back

Thanks
Darin