Sick Ghia (I Think)

[jon]

I recently built a CG using an AO-35 for iron and a 17x8x2 chassis. I followed the known layout (attached) and BOM (attached) and I am getting some odd results. The amp seems to be low on power. With the amp connected to my 2X12 oval port with JBL E120's it is quite loud (as expected with two very efficient speakers) but the distortion is a little harsh. With my sealed 1X12 loaded with a Private Jack it has very little volume and again it has harsh distortion. With the Volume below half the cleans are nice. The only deviations from a stock CG are the 100ohm screen resistors. I did this for ease of biasing.

I know that this amp has a mixed feeling around here, but I have listen to a few that just sounded glorious. Mine doesn't. I have attache a spread sheet to show the current draws of the power tube as a reference.

[IMG:600:800]http://i82.photobucket.com/albums/j250/Jhtjon/993A1BFC-D697-42A8-AC7A-C8198912FB07-1486-00000100E302B530_zps8bb16267.jpg[/img]

[IMG:600:800]http://i82.photobucket.com/albums/j250/Jhtjon/7A7E5EFB-198E-4BA3-8F53-AD17B17C3BA7-1486-00000100D738588E_zps13e92c8e.jpg[/img]

[IMG:600:800]http://i82.photobucket.com/albums/j250/Jhtjon/IMG_26351_zpsd26277fe.jpg[/img]

[xtian]

First, the obvious: What are the impedances for the two speaker cabs you tried?

[jon]

Both 8

[xtian]

Is the plate worksheet you posted YOURS?

Guess I'd like to see voltages from the rest of the amp.

[Bob S]

I'd check the dc resistance of your E120's
JBL of old used to mark speakers with incorrect resistances.
I've measured them at 6.7 to 8 ohms when they were marked 16 ohms.
Might explain a little of the volume puzzle.
You're right about speaker efficiency being a big factor too.

[jon]

Is the plate worksheet you posted YOURS?

Guess I'd like to see voltages from the rest of the amp.

Yes the plate work sheet is mine
both the private jack cabinet and the JBL cabinet ohm out at 7.6

I will post my voltages but they were all close when I fired the amp the first time.

[jon]

Voltages as requested

V1 Preamp 12AX7
1-150
2-NA
3-1.235
4-H
5-H
6-169.2
7-NA
8-1.01
9-H

V2 PI 5751
1-225
2-23.3
3-26.9
4-H
5-H
6-230
7-23.3
8-26.9
9-H

V3 Power
1-NC
2-G
3-8.89 Shared
4-H
5-H
6-NC
7-298
8-NC
9-294.7

V4 Power
1-NC
2-G
3-8.89 Shared
4-H
5-H
6-NC
7-298
8-NC
9-294.7

V1 5Y3
1-NC
2-5
3-NC
4-290
5-NC
6-290
7-NC
8-300.6 VDC

[jon]

anyone

[M Fowler]

Jon keep checking your work because I used that layout to build my CG and the amp works well.

Mark

[jon]

I scoped the amp this morning and here were my results:
Using a 2mV input and connecting the probe to the plate of the Power Tube here is what my forms looked like.

[JazzGuitarGimp]

That almost looks like crossover distortion. Are the output tubes biased warm enough?

[JazzGuitarGimp]

Ah, I just saw your plate dissipation worksheet. 70% seems reasonable.

[xtian]

So, divide and conquer. Insert your sine wave at the input of the PI.

[Davidg]

any chance the 100R screen resistors have anything to do with it.That seems awfully low and I think can cause crossover distortion I know KOC recommends adding screen resistors of at least 1k to any amp that didnt come with them-I think either SVT or some hiwatts didnt have them but am not sure.Just a thought as I have found I prefer higher value screen resistors of 2k or higher sometimes and this is a little easier on the tubes as well.

[DocJames]

-edited- I wasn't bringing anything useful to the discussion for your amp