Baising Create vc50

[Eddie P]

Hey all,
New to the forum. I have a create vc50. I must say this amp has probably been the most pain in the butt, most of the problem was cold soulder joints and bad pots, but I'm just about done with it. I've seen a lot of schenatics on this amp none that i have seen gives you a idea of the current on the cathode's of power tubes when the amp is at idle. does any one have a idea on a good starting point?
Eddie P

[marcoloco961]

Hey all,
New to the forum. I have a create vc50. I must say this amp has probably been the most pain in the butt, most of the problem was cold soulder joints and bad pots, but I'm just about done with it. I've seen a lot of schenatics on this amp none that i have seen gives you a idea of the current on the cathode's of power tubes when the amp is at idle. does any one have a idea on a good starting point?
Eddie P

What type of power tubes does it use? Duncan amps has a nice tube data program that lets you look up info on all sorts of tubes. The mA each tube can draw depends on many factors. B+ at Power tube plates and screen, primary impedance of the OT, etc.

Posting a copy of the schematic would be helpful.

[marcoloco961]

I can't find that schemo, but the other Crate Vc**** amps look like they use EL84's. If it's a 50 watt w/ el84's then you probably have 4 power tubes?


According to the spec sheet on the el84, in class AB push/pull with 250 volts on the plates and screens:

Plate current (zero signal) = 2x31mA
Screen current (zero signal) = 2X3.5mA

With 300 volt on the plates and screens:

Plate current (zero signal) = 2x36 mA
Screen Current (zero signal) = 2x4ma

This example was using a 8K primary impedance on the plates.

Most folks seem to ignore the screen current to add in a safety factor. So if you are running 4 el84 tubes in push/pull you will have two banks of two tubes. So at 250 volts approx 31 ma per tube or 62 mA per bank.

At 300 volts approx 36mA per tube or 72mA per bank.

If they are not EL 84's then you will have to let us know what they are.

[Firestorm]

VC50 runs EL84s hot (391V). "Safe" starting point might be 20-21mA. Or just use the recommended -15.6 volts on the grids.

[Eddie P]

Hey All thanks for the responds,
The amp is loaded with 4 EL84's as far as I know all the VC20's,30's and 50's watters were all loaded with EL84's. I think I'll go with the 20mA-21mA and start there. That would be 80mA-84mA sounds like that going to be on the cold side should be a great starting point. I have builded a few SE EL84's and bias them around 40mA. I'm the same way I like adding the screen in the bias for the safety net. The plate voltage is pretty hot but I have seen them hotter, one of my SE amps has 425v on the plates and it's been rocken since 2006 on the same tube. One thing that really confuses me.How does Create sell these as 50 watters but by the spec's max watt on the plate is 12watts. 12 * 4 should't that be 44 or am i missing something here LOL. thanks again.
Eddie P

[marcoloco961]

Hey All thanks for the responds,
The amp is loaded with 4 EL84's as far as I know all the VC20's,30's and 50's watters were all loaded with EL84's. I think I'll go with the 20mA-21mA and start there. That would be 80mA-84mA sounds like that going to be on the cold side should be a great starting point. I have builded a few SE EL84's and bias them around 40mA. I'm the same way I like adding the screen in the bias for the safety net. The plate voltage is pretty hot but I have seen them hotter, one of my SE amps has 425v on the plates and it's been rocken since 2006 on the same tube. One thing that really confuses me.How does Create sell these as 50 watters but by the spec's max watt on the plate is 12watts. 12 * 4 should't that be 44 or am i missing something here LOL. thanks again.
Eddie P

The pre amp tubes add a few extra watts to the equation. But it still is probably not a full 50 watts of power. Only thing I see you missed is the 12 * 4 = 44.

Sorry, it's the wiseguy in me that had to point that out. LOL...

[martin manning]

The 12W per tube is max plate dissipation, not output power. 4x EL84 at that Va gives (on paper, at the annodes, at clipping) ~67W.

[Eddie P]

Hey All thanks for the responds,
The amp is loaded with 4 EL84's as far as I know all the VC20's,30's and 50's watters were all loaded with EL84's. I think I'll go with the 20mA-21mA and start there. That would be 80mA-84mA sounds like that going to be on the cold side should be a great starting point. I have builded a few SE EL84's and bias them around 40mA. I'm the same way I like adding the screen in the bias for the safety net. The plate voltage is pretty hot but I have seen them hotter, one of my SE amps has 425v on the plates and it's been rocken since 2006 on the same tube. One thing that really confuses me.How does Create sell these as 50 watters but by the spec's max watt on the plate is 12watts. 12 * 4 should't that be 44 or am i missing something here LOL. thanks again.
Eddie P

The pre amp tubes add a few extra watts to the equation. But it still is probably not a full 50 watts of power. Only thing I see you missed is the 12 * 4 = 44.

Sorry, it's the wiseguy in me that had to point that out. LOL...

Well I did miss that now that i had some sleep it's 48 LOL guess 48 is close enough for goverment work and round it up to 50 LOL

[Eddie P]

The 12W per tube is max plate dissipation, not output power. 4x EL84 at that Va gives (on paper, at the annodes, at clipping) ~67W.

Thanks Martin I never thought these little bottles could put out that much power. I looked at plate dissipation and power watts as the sane thing. Looks like i need to look at the books again. LOL