carlsbro 50 top clone help !!!!

[blindboybenton]

]Hi

Ive posted a few bits about this one before. so i thought i would start a fresh thread

Im trying to learn more about valve amps. Ive been given a old carlsbro cs60 pa head with a burnt out tag board. So i thought i would strip and rebuild/convert to a 50 top
as i used to have one years ago and really like it
Ive got the amp working but i have a lot of noisy. i can see the noisy on the scope, but im unsure how to read and understand. Been trying to do all the work on this amp on my own without much help. so i can learn.

If you plug a guitar or bass into the amp. You can hear the guitar well. I can describe it like. If you remove the noise the sound of the guitar would be perfect. It seem to be some hum along with a high pitched rattle.. sound clips below


I plan to use the GZ34 but at present in using solid state. The original tx is linear type.. Ive tried both ways of wiring up.. Not sure what the name is for it, wired now 3 wire
All parts are new except tx's and valves. The valves have been swapped out many times. The amp has been copied from the schematic
Carlsbro 50 Top Amp Schematic

Here are some voltages all taken with no signal ( Schematic attached )
Heaters 6.02v ac
HT voltage 560v ( under full loading drops to 440v
biasing is variable and is ok 40ma per tube ( no bias tap on tx so coming off secondary ac out )

V1 pin 8 = 1.2v - pin 3 = 1.2v - Pin 1 = 181v - Pin 4 = 174
V2 pin 1 = 290v - Pin 6 = 280v - pin 3/8 = 86v pin 2/7 = 52v

At present i have not got all the parts for the tone control. So ive connected the 100k thats connected to 0.047uf @ pin 4 of v1 to one side of the volume pot.
seem to be working ok

Scope reading from the anywhere on the signal path ( without signal ) ( 2v per div and 2ms) with vol turn up 1/4
Scope reading from the decoupling caps @ EL34 with a signal from gen
Scope reading from the pin 4 of v1 volume off


Voice0003.aac sound clip of the amp with a bass playing ( The mobile can't really catch the noise of this properly)
Voice0001.aac sound clip without guiar

If anybody has some idea. Please any help would be appreciated.

Bassman1965

[pdf64]

Could you review the 2nd paragraph of your post, the wording seems rather confusing?
Could you describe the noise, or record it and upload it somewhere, eg youtube?
Good quality photos of the amp's board and wiring would be helpful.
Pete

[blindboybenton]

ive recorded some sounds via mobile. no decent camera so cant get pictures of the board just yet

hope they can help

bassman1965

[olddawg]

With the input terminated use your scope to measure the ripple voltage on your power supply taps.

[blindboybenton]

Do you think its power supply causing this ?

Ive checked the psu with the scope and i don't think its coming from the psu. But im not the best of reading a scope

[olddawg]

Do you think its power supply causing this ?

Ive checked the psu with the scope and i don't think its coming from the psu. But im not the best of reading a scope

Don't know. I can't get your clips to play on my iPhone. But it is a simple matter of looking at your power supplies with the scope AC coupled to measure the ripple. If you measure the DC make sure you use a 10:1 probe. Also check the output for oscillation. Did you make a new turret/tag board? Are you following a known good layout? You have a scope. Do you have a dummy load? You can download a signal generator ap for fee.

[blindboybenton]

Hi oldDawg

Yes i do have a dummy load. I used all new parts inc tag board. Ive never made a amp before on tag board. You said " Are you following a known good layout"
No i guess. I just got some paper and slowly drawn out the circuit on the paper. So all the connections for say V1 are together. I only have a shit old mobile at present which takes shit pictures . I have tried adding more caps and it makes no different

Its the spikes you see on the scope that i don't understand. The sound clips are ok when i check them on my pc. Im happy to email them to you,?. Im going to make another psu today on a old chassis and try that. I feel im so close to rapping this up, But im pulling my hair out. I aint got much left as it is

Bassman1965

[pdf64]

I just had a listen and there's an awful lot of buzzy interference there.
Is a known good instrument and lead being used?
If so, has the heater circuit got a ground reference, eg centre tap to ground, or artificial centre tap using 2 x 100 ohm resistors?
Is the chassis properly grounded?
Pete

[blindboybenton]

Hi Pete

yes the guitar is good its my best bass lol .Ive checked the heater ct. all connected ok. Checked over the amp with the multi meter all seems ok. The noise is there without any signal at the input

Do you think its the heaters ? being new to this if i scope the heater rail , should i see a clean AC signal. All the heater connections are using twisted cable

Also. Not sure if i mentioned this. But the output tx starts to squeal if you turn up the volume with or without the signal

scope reading without signal on the heaters

bassman1965

[pdf64]

'the output tx starts to squeal if you turn up the volume with or without the signal.
That may be due to non-ideal layout, lead dress - hence the request for photos of innerds.

OK, it's not so much poor heater lead dress that causes such buzz; what's the Vac between each leg of heaters and the chassis / ground?
If ~3.2V then try (yet) another ECC83 in V1 then V2.
Pete

[blindboybenton]

Hi Pete

Thanks for your help

Does the scope picture of the heaters look ok?
I'm going have to borrow a camera. Ive looked at the layout all seems to be the same sort of layout like a marshall jcm800. All wires are short going v1/2..The first version of the board i did . I had trouble tracing the problem . So i stripped it all and re worked it all with all link wires on the top side of the tag board

while im waiting for the camara is there anything that spring to mind on the layout.

All this has put me off doing clones. But the next project is waiting to get its board.... God help me

Bassman1965

[pdf64]

The heater signal looks awful - can you check the Vac between each leg of heaters and the chassis / ground?

[blindboybenton]

Many thanks too you all for the help

The amp is done working really well

It turned out to be a bad Power tx. In all the playing around with it. The CT for the heaters has gone o/s
I was checking the heater voltage like pete said i was getting a strange read from ground to one side of the heaters. I checked the heater CT and its o/s.
It was not when i started this project.

I had a tx made for a 20watt amp. used the heater rail off that its fine.

My next question is

Is there a mod i can do to sort out the CT issue with the original tx ? if not i may get a heater tx

Bassman 1965

[olddawg]

Are you saying that the heater center tap is open? Strange. Are you absolutely sure it is a center tap and not a different tap? You could always try an artificial center tap as mentioned before with two 100 ohm resistors and float the old center tap. You could also use a separate small filament transformer if everything else is fine. I would float all of your PT secondaries, take some serious measurements,and be sure before you do anything though.

[blindboybenton]

Thanks OD

Ive checked it open it is .. Ive found the 100R resistor mod and done that. working well. still got work to rap up. But im on the home straight

bm1965