ODS 6V6 build?

[ToneMerc]

You need to hang a 150k bleeder off of B5+ to draw down the preamp voltage rail, without it V1-V3 wil be about 22-25V too high. Also bias should be adjusted to around 32-35mA.

TM

Thanks TM, i just put it in, but my issue still is upstream at the 6L6gc plates, or what I believe to be the PT. My plate voltage, at 33mA is 480VDC. Increasing the bias to 45mA or so only drops Vp a bit.
Even if I was looking for 480V plates, are these amps usually biased that cold? And are there voltage charts around for these 50W D'lites?

The bleeder only affects the preamp rail and should be in the circuit and yes that is typically the bias range of a Dumble ODS, you can season to taste. So, now you are about 15-25V too high on the plates.

On my 50W D'lite I'm running around 465-470, but that is by design.

Here's some typical voltage ranges that you can kind of ball park

PI: 275-310
Plates: 445-465

TM

[Smokebreak]

I've got PI : 320-325, V1: 205,211 V2 : 208,225 . I'm thinking of swapping PT. The amp sounds good, but i have nothing to compare it to, and haven't got to crank it yet. I guess I could keep it but I'm right at 450V on the 2nd cap after the reservoir caps, so I'd need to bump that up.

[ToneMerc]

I've got PI : 320-325, V1: 205,211 V2 : 208,225 . I'm thinking of swapping PT. The amp sounds good, but i have nothing to compare it to, and haven't got to crank it yet. I guess I could keep it but I'm right at 450V on the 2nd cap after the reservoir caps, so I'd need to bump that up.

You could also put a 125-150R 25W resistor between the HT CT(red/yel) and ground to pull down volatge as well.

TM

[rogb]

I've got PI : 320-325, V1: 205,211 V2 : 208,225 . I'm thinking of swapping PT. The amp sounds good, but i have nothing to compare it to, and haven't got to crank it yet. I guess I could keep it but I'm right at 450V on the 2nd cap after the reservoir caps, so I'd need to bump that up.

You could also put a 125-150R 25W resistor between the HT CT(red/yel) and ground to pull down volatge as well.

TM

Or a Zener diode that bolts through the chassis. I used this one @ 30v

(SDI) 1N3324RBJAN Zener Diode

www.surplussales.com

[JazzGuitarGimp]

I've got PI : 320-325, V1: 205,211 V2 : 208,225 . I'm thinking of swapping PT. The amp sounds good, but i have nothing to compare it to, and haven't got to crank it yet. I guess I could keep it but I'm right at 450V on the 2nd cap after the reservoir caps, so I'd need to bump that up.

You could also put a 125-150R 25W resistor between the HT CT(red/yel) and ground to pull down volatge as well.

TM

That's a good way to fry the PT. 465V across 125R = 3.72 Amps. Thar resistor would need to dissapate a little over 1,700 watts. Or am i missimg something?

[ToneMerc]

That's a good way to fry the PT. 465V across 125R = 3.72 Amps. Thar resistor would need to dissapate a little over 1,700 watts. Or am i missimg something?

Lou, I was always under the impression that it's based on HT current draw through the HT winding, not DC voltage post rectifier.
In this case if the HT winding sees say 250mA of total draw and he wanted to drop the voltage 30V, would the wattage rating need to be at least (.250 x30) 7.5 watts?

Wouldn't a zener diode have the same issue as well?

I dropped voltage on a harp build and it worked and I'm pretty sure Gary has done it before as well.

https://tubeamparchive.com/viewtopic.php?t=13594

TM

[JazzGuitarGimp]

That's a good way to fry the PT. 465V across 125R = 3.72 Amps. Thar resistor would need to dissapate a little over 1,700 watts. Or am i missimg something?

Lou, I was always under the impression that it's based on HT current draw through the HT winding, not DC voltage post rectifier.
In this case if the HT winding sees say 250mA of total draw and he wanted to drop the voltage 30V, would the wattage rating need to be at least (.250 x30) 7.5 watts?

Wouldn't a zener diode have the same issue as well?

I dropped voltage on a harp build and it worked and I'm pretty sure Gary has done it before as well.

https://tubeamparchive.com/viewtopic.php?t=13594

TM

Okay, I misunderstood. You're on-point, TM. It looked like you were suggesting connecting the resistor from HT to ground to pull the voltage down by adding additional load. Which might work, if the transformer was over-speced for the amp, but the resistor would need to be much higher in value.

Carry on!
Lou

[Smokebreak]

I ordered a classictone PT 650CT, and will eventually send back this Hammond, and hopefully get another that produces appropriate voltages, so we'll shoot em out.

Can we talk ground scheme for a minute? A couple of builds ago I told myself I'd never use a bus again, as I've been mostly unsuccessful with them in higher gain builds. I followed the documented scheme for the D'lite nonetheless, even though I remember thinking "that's a LOT of chassis connections". I've been playing buckers with this thing since fire up , and it was quiet enough. I used a tele last night and good golly it was super noisy, especially in OD mode. My test is always with everything cranked(internal trimmer around 2oclock). I know there's always gonna be some sc noise, but this is unbearable. I've got v1 a,b and v2 b shielded, too. I didn't shield MV to PI, but I can't see that quieting things that much.

Has anyone else had similar results, and implemented a star with better results in this type of build?

Only slightly off topic, when pushing wires around, I found that moving the PAB connection away from the chassis crease(and the rest of the tone stack wires) actually changed the tone and made the amp brighter. That's the only affect chopsticking the amp had.

[ToneMerc]

Can we talk ground scheme for a minute?

No need to reinvent the wheel here, look in the files section at the Dumble layouts and follow the way HAD did it.

TM

[Smokebreak]

I've followed this ground scheme implicitly , which seems based on the original schemes. I'm still getting way too much hum. With no input, the amp is quiet.ive removed the shields to test, and that doesn't make the hum worse.reattaching the v1b shield actually makes it a bit noisier. I'll keep poking around.[img1247]http://bad-domain/DumbleLite/DumbleLite16x8Layout.JPG[/img]

[M Fowler]

I built this circuit into a Princeton Reverb size chassis/cab and it was dead quiet so I suggest looking for something out of place.

[Smokebreak]

Allright, I'm going a little crazy over here. To recap, I had this Hammond in my 50W D'lite: http://www.tubesandmore.com/sites/defa ... t290dx.pdf.
Its 200mA 650CT. I was getting 720VAC across the PT secondaries loaded with 3 12ax7 and 2 6L6GC.

I was convinced it was defective, and ordered this Classictone: http://www.classictone.net/40-18073.pdf
Exact same specs.

I just put in that one, and am getting the exact same secondary readings, with same plates as the Hammond- 475VDC

I have used 3 different, albeit cheap, meters and they all read the same. 1 is brand new.
My only deviation from schematic is that I put the reservoir caps before the standby.
Amp works great, sounds good, just hums a lot with single coils and everything cranked.
While the PT is probably half loaded at idle, even if it was unloaded I shouldn't be getting 720VAC across secondaries, I don't think.
Wall voltage is 122VAC.
No way I could get two different transformers that are faulty, right?
Any Ideas?

[John_P_WI]

Are you using a bridge rectifier or a full wave rectifier with the center tap grounded? A full wave with the center tap grounded should give around 460v give or take.

[Smokebreak]

Are you using a bridge rectifier or a full wave rectifier with the center tap grounded? A full wave with the center tap grounded should give around 460v give or take.

I'm using full wave rectifier, which is functioning properly. My dilemma is that this is the second 325-0-325 PT I've put in the amp in a week, and they are giving 360-0-360 at the PT secondary/rectifier junction.
I've boiled it down to :
-2 faulty transformers(unlikely)
- user error(highly likely)
I'm just trying to figure out what sort of wiring mistake could cause this, or if the PTs weren't loaded enough to bring the AC down to advertised specs. I find the latter unlikely as well.

[wjdunham]

Are you using a bridge rectifier or a full wave rectifier with the center tap grounded? A full wave with the center tap grounded should give around 460v give or take.

I'm using full wave rectifier, which is functioning properly. My dilemma is that this is the second 325-0-325 PT I've put in the amp in a week, and they are giving 360-0-360 at the PT secondary/rectifier junction.
I've boiled it down to :
-2 faulty transformers(unlikely)
- user error(highly likely)
I'm just trying to figure out what sort of wiring mistake could cause this, or if the PTs weren't loaded enough to bring the AC down to advertised specs. I find the latter unlikely as well.

One simple thing to check, depending on the finish on your chassis - did you make sure to grind the all of the ground connections down to the metal and ensure good contact, sure sounds like you've got some weak grounds both in terms of the noise issues and voltages. I've used both those transformers with the multi-point Dumble style grounding scheme and the amps are dead quiet and voltages are right on the specs.

Bill

www.sebagosound.com