#102 Started. Checking in

[martin manning]

Taylor, yes definitely sort the FET power supply out first. I was having a tough time understanding your V1 voltages being higher than V2, and that could very well explain it. The FET booster will draw ~1.7mA...as much as another triode!

[ToneMerc]

I don't have the board wired right, I may have mixed up two resistors, which might explain the voltages at the tube. The FET input is not working right, it makes sound but sounds like its starving for dc. I will take a better look at the source resistor. I'm gonna get that working right before messing with the dropping string

Taylor, IIRC those 3 pre-bent lead resistors should have a dark colored 10K and two light colored ones which were either 22K or 27K and a 2.2K. It looks like R5 and R4 are swapped. R5 should be the 22 or 27K and R4 the 10K.

TM

[amplifiednation]

I don't have the board wired right, I may have mixed up two resistors, which might explain the voltages at the tube. The FET input is not working right, it makes sound but sounds like its starving for dc. I will take a better look at the source resistor. I'm gonna get that working right before messing with the dropping string

Taylor, IIRC those 3 pre-bent lead resistors should have a dark colored 10K and two light colored ones which were either 22K or 27K and a 2.2K. It looks like R5 and R4 are swapped. R5 should be the 22 or 27K and R4 the 10K.

TM

Thanks Tonemerc. i was going to ask you about those. I found a post of yours that indicated you had found some values that differed from the layouts. I pulled those three resistors from the board.

I'll get them back in tomorrow if the wife isn't on anti-amp patrol...which she surely is today after building three cabinets and messin with the 102.

[ToneMerc]

I don't have the board wired right, I may have mixed up two resistors, which might explain the voltages at the tube. The FET input is not working right, it makes sound but sounds like its starving for dc. I will take a better look at the source resistor. I'm gonna get that working right before messing with the dropping string

Taylor, IIRC those 3 pre-bent lead resistors should have a dark colored 10K and two light colored ones which were either 22K or 27K and a 2.2K. It looks like R5 and R4 are swapped. R5 should be the 22 or 27K and R4 the 10K.

TM

Thanks Tonemerc. i was going to ask you about those. I found a post of yours that indicated you had found some values that differed from the layouts. I pulled those three resistors from the board.

Taylor, here'a great thread about the FET bias. As you read through it, you'll see values in those ranges. My results have been very similar and thus the reason for those values.

https://tubeamparchive.com/viewtopic.ph ... ht=27k+fet


TM

[amplifiednation]

Ok...got the FET working. Sounds a little harsh, almost 'tinny' maybe i can dial that out? I haven't played with the trimmer yet.

Preamp

v1a/v1b - 205.7/212.8
v2a/v2b - 200.5/205.6
v3a/v3b - 291.5/289.5

B3 - 433
B4 - 327
B5 - 320

right now i have a 22k from B3 to B4 and a 2.2K from B4 to B5. I got those values off the 183 PS layout since I went with the precision power supply.

Is B5 too high or am I just too high on the plates?

[martin manning]

What's the voltage at the top and the bottom of the FET's 10k drain resistor? The top should be 2x the bottom to approximately center bias. The bias will affect the tone and current draw. FWIW, my FET sounded very bright initially, and then settled down.

If the FET bias is good, then you probably do want to increase the 2k2 B+4-5 dropping resistor to drop another 15-20V at V1's plates, shooting for 180-190. Try swapping V1 and V2 to see what the effect on the plate voltages is.

[amplifiednation]

What's the voltage at the top and the bottom of the FET's 10k drain resistor? The top should be 2x the bottom to approximately center bias. The bias will affect the tone and current draw. FWIW, my FET sounded very bright initially, and then settled down.

If the FET bias is good, then you probably do want to increase the 2k2 B+4-5 dropping resistor to drop another 15-20V at V1's plates, shooting for 180-190. Try swapping V1 and V2 to see what the effect on the plate voltages is.

The FET voltages are not correct. From my understanding the top of the 10k should be 20v, then 10v at the bottom. I have 4.5v at the top, 3.5v at the bottom.

I am using a 2.2k next to the 150k resistor. I have 320v going into the 150k

I'm not sure how I got this far off..

Swapped tubes

v1a/v1b - 205.7/212.8 new 195.3/200.2
v2a/v2b - 200.5/205.6 new 207.8/215

So those seem a bit better. FET is still 4.5v/3.5v on the drain

[martin manning]

Did you miss-read 2k2 for 22k? That would probably be about right, 22-27k.

[amplifiednation]

Did you miss-read 2k2 for 22k? That would probably be about right, 22-27k.

I've got red red red red on the resistor adjacent to the 150k, 2.2k

again, not a real pretty shot of my work...but maybe there is something i missed. the resistor at the bottom, the dark one...that is the other value that differs from the layout. Looks like red yellow red which would only be 2.4k. Is that my problem?

the resistors you can't see are the 10k drain (verified) and the 1M (verified)

[martin manning]

2k4 for the source resistor is at least close, I think I'm using 2k. Why don't you try powering the FET with two 9V batteries. Disconnect the B+ from the 150k, and put 18VDC at the top of the 10k. Then look for 9VDC at the FET's drain. If you don't get that then you need to adjust the source resistor.

[amplifiednation]

Isn't one of these resistors supposed to be around 22 or 27k?

I'll try the battery trick...probably won't be until tomorrow though.

My voltages seem better changing the tubes around. big swing there.

[martin manning]

Isn't one of these resistors supposed to be around 22 or 27k?

I'll try the battery trick...probably won't be until tomorrow though.

My voltages seem better changing the tubes around. big swing there.

Yea, the one on the other side of the electrolytic from the 150k. 22k is close enough for now. Get the bias set using a battery to determine the source resistor value, then you can tweak the 22k to get 20V at the top of the 10k.

[amplifiednation]

Isn't one of these resistors supposed to be around 22 or 27k?

I'll try the battery trick...probably won't be until tomorrow though.

My voltages seem better changing the tubes around. big swing there.

Yea, the one on the other side of the electrolytic from the 150k. 22k is close enough for now. Get the bias set using a battery to determine the source resistor value, then you can tweak the 22k to get 20V at the top of the 10k.

Thanks Martin. I will update asap

[ToneMerc]

Taylor you have resistors R3-5 out place. I'm pretty sure it was a 27K that I sent you as I'm out of the 22K. You should have a 2.2K, 10K and 27K.

TM

[amplifiednation]

Taylor you have resistors R3-5 out place. I'm pretty sure it was a 27K that I sent you as I'm out of the 22K. You should have a 2.2K, 10K and 27K.

TM

TM,
Its funny I needed this picture to figure out the fet. Haha thanks to you and Martin I might have this amp at 100% soon.

Now that i dropped v1 ~10v below v2 I hope I'm good with the voltages. We'll see once the fet is biased right. The amp is sounding great.

Bright switch is a little much for me I am going to lower that cap. I will rewire some of the areas to get out burnt pots