rectifier circuits

[jestaudio]

Some one correct me if i,m wrong but as i understand it using a diode rectifier circuit the dc voltage will be approx 1.4 times greater than the ac on the ht side, if thats correct then i would need to adjust the dropping string on a marshall 18 watt tmb circuit to allow for lower dc voltage. the question then is not knowing the amperage the circuit is going to pull how do i go about it,
Sorry for the newb question

[LeftyStrat]

That's going to be the voltage without a load. Figure around 1.2 with a load applied. What kind of voltage are you looking to drop?

[martin manning]

You want to switch from EZ81 to SS rectifier, and hit the same voltages, right? Do you know how much voltage you will have to drop? It will be a guess to start, but then you can bring it in with a second pass. I'd estimate about 95mA, and something like 30V drop for the vacuum rectifier, so that would be 315 ohms. Standard values are 300 or 330. Power is 0.095^2 x 315, or 2.84W. A 5W resistor should do, or use a 10W if you have room.

[jestaudio]

That's going to be the voltage without a load. Figure around 1.2 with a load applied. What kind of voltage are you looking to drop?

that figures going from the readings i,ve got ith the trannie still in the original chassis,
i,m looking at doing a varient on the 36 watt TMB marshall circuit, the original specs call for a 290-0-290 trannie, i think that would probaly give 600 or so volts before being rectified so with the 300 rectified voltage available i would be needing to use a voltage doubler befoer i,m anywhere close, fel free to point out the holes in my theory but i suspect the trannie may not have enough bals to supply the ampage needed with a doubler

[ToneMerc]

i,m looking at doing a varient on the 36 watt TMB marshall circuit, the original specs call for a 290-0-290 trannie, i think that would probaly give 600 or so volts before being rectified so with the 300 rectified voltage available i would be needing to use a voltage doubler befoer i,m anywhere close, fel free to point out the holes in my theory but i suspect the trannie may not have enough bals to supply the ampage needed with a doubler

I'm confused here, what exactly are you trying to do? I built an 18W 1974X head earlier in the year using a 290V PT. Sounds like you are trying to reinvent the wheel, please provide more info.

What's this "trannie in the original chassis"? Do you want to use an existing PT that you have on hand?

TM

[jestaudio]

i,m looking at doing a varient on the 36 watt TMB marshall circuit, the original specs call for a 290-0-290 trannie, i think that would probaly give 600 or so volts before being rectified so with the 300 rectified voltage available i would be needing to use a voltage doubler befoer i,m anywhere close, fel free to point out the holes in my theory but i suspect the trannie may not have enough bals to supply the ampage needed with a doubler

I'm confused here, what exactly are you trying to do? I built an 18W 1974X head earlier in the year using a 290V PT. Sounds like you are trying to reinvent the wheel, please provide more info.

What's this "trannie in the original chassis"? Do you want to use an existing PT that you have on hand?

TM

sorry for the confusion, i have a old laney which i,m gutting and want to keep most of the parts or at least as much as is practical, the laney trannie kicks out 240 ish across the ht no centre tap, the scematic calls for 290-0-290 so i,m trying to find a way to use a under sized trannie

[Phil_S]

The solid state rectifier will give you the maximum DC output. If you have 240-240 without a load, I think you can figure 240 * sqrt(2) * ~.95 = ~320. That might be a little on the high side, but not that much so, IMO. The OT will drop a bit. I think you'll see under 320 at the plates. An EL84 can handle this. You can add a sag resistor to simulate the tube rectifier. If there is a concern about voltage, use JJ or Sovtek EL84M. You may need to increase the downstream dropping resistors to make the preamp right, but, really, I don't think any of it is much of a concern.

[jestaudio]

The solid state rectifier will give you the maximum DC output. If you have 240-240 without a load, I think you can figure 240 * sqrt(2) * ~.95 = ~320. That might be a little on the high side, but not that much so, IMO. The OT will drop a bit. I think you'll see under 320 at the plates. An EL84 can handle this. You can add a sag resistor to simulate the tube rectifier. If there is a concern about voltage, use JJ or Sovtek EL84M. You may need to increase the downstream dropping resistors to make the preamp right, but, really, I don't think any of it is much of a concern.

its actually 120-120 measured each ht to ground and 240 across the ht leads

[LeftyStrat]

Yeah, use a bridge rectifier and you should be in the ballpark.

The original 290-0-290 unloaded is 410, but there is a voltage drop of the rectifier.

From 240, you'll get 340 unloaded with a bridge rectifier, with no rectifier voltage drop.

[ToneMerc]

i,m looking at doing a varient on the 36 watt TMB marshall circuit, the original specs call for a 290-0-290 trannie, i think that would probaly give 600 or so volts before being rectified so with the 300 rectified voltage available i would be needing to use a voltage doubler befoer i,m anywhere close, fel free to point out the holes in my theory but i suspect the trannie may not have enough bals to supply the ampage needed with a doubler

I'm confused here, what exactly are you trying to do? I built an 18W 1974X head earlier in the year using a 290V PT. Sounds like you are trying to reinvent the wheel, please provide more info.

What's this "trannie in the original chassis"? Do you want to use an existing PT that you have on hand?

TM

sorry for the confusion, i have a old laney which i,m gutting and want to keep most of the parts or at least as much as is practical, the laney trannie kicks out 240 ish across the ht no centre tap, the scematic calls for 290-0-290 so i,m trying to find a way to use a under sized trannie

Ok, got ya now. If you want to do a build in the typical voltage range you will need a B+ in the 340-360V loaded area and you right, that PT won't cut it. However, could you build one with a 290V B+ under load sure, but why?

My canned answer is bite the bullet and obtain the correct PT transformer for the application as you can always down the road implement VVR/power scaling, but you can't increase the B+.

TM

[ToneMerc]

its actually 120-120 measured each ht to ground and 240 across the ht leads

So that's even less...... you know what you have to do.

TM

[LeftyStrat]

So what was the original Laney amp running? I can't imagine a tube amp using 120-120.

[LeftyStrat]

Looking at your other post, the Laney was already using el84x4, that transformer should about right. Are you measuring AC voltage? That PT has to be putting out more than 120v.

Is the PT still in the original circuit? Can you measure DC after the rectifier?

[cbass]

So what was the original Laney amp running? I can't imagine a tube amp using 120-120.

From what I am understanding Its using a bridge rectifier. Wich wiil double the voltage than if it was just fullwave.So should be about 335vdc unloaded

[jestaudio]

So what was the original Laney amp running? I can't imagine a tube amp using 120-120.

From what I am understanding Its using a bridge rectifier. Wich wiil double the voltage than if it was just fullwave.So should be about 335vdc unloaded

not far off, about 320 unloaded