Jcm800 2203 questions

[giordydiver]

Hi, I am a new user of the forum and write from italy.

I'm building my first amp,a jcm800 2203 by following the manual metroamp 100w master volume, with zero loss fx and sir34(also metroamp schematic) . I'm using 6550 power tube but I have a question on the bias. 6550 EH are 42w tubes and the plate voltage is 480v , now following the rule of manual metroamp:
(42x0.7) / 480 = 58mA, is ok this value or I'm wrong something?
Sorry but is my first amp


Some photo, work in progress

Advice about the work are accepted

[Haze13]

You should talk to the company who made this Output Transformer. The problem is that this is a high value for a typical guitar amp. On the Weber site there is a calculator for these things and some tables for various tubes. For a 6550 at 475 the bias is 52mA and they call it HOT. The average bias is 44mA. For a Class AB1 this (58mA) will be a good value, but for an AUDIO amp.
If you power transformer can supply this amount of current + 75% of it and your OT is rated for 250mA (58x4=232 in your case) of a DC current - then there can be no problem... If not, than it must be lower.

[giordydiver]

ok so if I understand correctly, 58 but it is theoretically correct but depends OT right? In the event that OT maximum current is 250ma use this value or better around the 48ma ?
other question, I have followed the guide metroamp and I changed resistors grid from 220k to 82k, NFB attacked at 4 ohms and resistance after the diode from 27k to 15k. Now I have seen that are most used replace to 150k and some use NFB 8ohm. someone can explain difference in term of sound?

[giordydiver]

So this is my pt and ot spec
PT: Primay 230 V
Secondary 175 – 0 – 175 V current 400 mA
3,15 – 0 – 3,15 V current 7,5 A each filament
98 V 50 mA

OT : 1700 Ohm
4 – 8 – 16 Ohm
100w
choke 5h 150ma

What do you thing?

[neikeel]

Haze13 is spot on.

The stock Marshall OT (and PT for that matter are not up to delivering 170w with any reliability, you should, in this context, think of the 6550 as no more than a 30w tube (max) and bias accordingly. Similarly you should decrease your grid leak resistors (aka bias splitters) on the PI from 220k (as used on EL34s) down to less than 100k (Marshall used 82k). You will lose some bass when you lower these so bigger PI output caps help to compensate (try 0.047uF - 100nF range).

[giordydiver]

one moment ,I am lost. So ,I use this guide http://www.metroamp.com/downloads/100W_ ... WNLOAD.pdf and for 6550 they say ( as I do) replace 220k grid resistor to 82k', nfb tap to 4 ohm, and 27k before diode to 15k . Is correct? In this way what happen to sound?

Now the bias, EH are 42w, think to tubes as 42w or 35w/30w?

[neikeel]

one moment ,I am lost. So ,I use this guide http://www.metroamp.com/downloads/100W_ ... WNLOAD.pdf and for 6550 they say ( as I do) replace 220k grid resistor to 82k', nfb tap to 4 ohm, and 27k before diode to 15k . Is correct? In this way what happen to sound?

Now the bias, EH are 42w, think to tubes as 42w or 35w/30w?

Why are you lost?

The above is correct, however if you do not increase the size of the PI output couplers you may find that there is a little loss of low end.

Yes, I strongly suggest that you bias as 30w tubes, not 42 or 35.

[giordydiver]

ok perfect! so for PI output cap is referred to two cap 0.022uf from v3 plate? what happen if I use tube as 35w or 42w? there is possibility to damage ot?

[neikeel]

ok perfect! so for PI output cap is referred to two cap 0.022uf from v3 plate? what happen if I use tube as 35w or 42w? there is possibility to damage ot?

Yes - you replace the 0.022uF with 0.1uF (like a super bass) it compensates for loss of low end

Yes if your tubes are at all tired they are likely to fail and take other components with them.

[giordydiver]

Ok thanks,
At the last, what is the difference betwen 150k and 82k as grid resistor? What change in term of sound

[giordydiver]

to max current is 250 ma. can I biasing as 35w tube? about 48 50ma

[ezs]

70% 42w w/6550 is too hot imo. I also bias 6550 @ 55% 35w max dissipation, but that's me.

[ezs]

Ok thanks,
At the last, what is the difference betwen 150k and 82k as grid resistor? What change in term of sound

Larger the bias splitter, will allow more bass.

[giordydiver]

Ok thanks,
At the last, what is the difference betwen 150k and 82k as grid resistor? What change in term of sound

Larger the bias splitter, will allow more bass.

Ok thanks, so if i use 150k and not 82k i must change nfb to 8 ohm or not?
What is the best solution in term of sound and safety?

[Haze13]

70% for 6550 is hot? Look at the Ampeg SVT! One tube gives 50 watt of power. 300w for 6 - 6550's/ That's HOT