High voltage readings on new build

MattPete · Post by **MattPete** » Tue Apr 03, 2007 9:55 am

Right now I'm at the step of installing the preamp tubes and checking the voltages. The voltages for the power tubes look fine (~480), as do the heater voltages on pre and power tubes (3.0 VAC). Poweramp pins 5 read at -38 VDC.

The problem is pin 1 of the preamp tubes:

V3-1 245
V2-1 242
V1-1 24x (I'm at work and don't remember).

These readings are with the preamp tubes in, power tubes out.

V3-1 isn't too bad, as it's within 20% of the voltage chart. However, V2-1 and V1-1 aren't dropping proportionally, and ideally should be around 160 and 150. Any ideas what would cause those high voltages?

MattPete · Post by **MattPete** » Wed Apr 04, 2007 8:44 pm

Here are some new clues. Not only am I getting high voltage on some preamp pins, but no voltage on some others:

Here are some new clues. Not only am I getting high voltage on some preamp pins, but no voltage on some others:

V1-1 241
V1-2 0
V1-3 0
V1-6 242
V1-7 0
V1-8 0

V2-1 244
V2-2 0
V2-3 0
V2-6 268
V2-7 245
V2-8 0

V3-1 241
V3-2 0
V3-3 0
V3-6 247
V3-7 0
V3-8 0

After tracing through the circuit from the pins that have no voltage but should have quite a bit (V2-8, V3-2, V3-3), it seems that I'm not getting any juice to the resistor on the board that connects the presence knob to the 8 ohm tap.

MattPete · Post by **MattPete** » Wed Apr 04, 2007 10:08 pm

Scratch some of those readings: I changed the HT fuse, and now the readings on V1-V3 that were in the 240 range are now around 480. The readings for the power amp look fine.

I'm giving up for the night. In the mean time, here are some photos. Maybe someone can spot something.

thanks in advance...
-Matt

novosibir · Post by **novosibir** » Thu Apr 05, 2007 4:05 am

You can't get correct readings and on some pins nothing to read w/o all the tubes in the amp. The tubes cause a voltage drop in the voltage supply rail and the tubes are connecting some pins in the sockets, where you now of course doesn't have any reading!

Larry

MattPete · Post by **MattPete** » Thu Apr 05, 2007 7:35 am

This is step 17.

In step 16, I took readings (heaters and pin 5 of powertubes) and everything looked OK.

So, now in step 17, I insert the preamp tubes and take readings, and compare them to the values in the voltage chart. So the readings above are with the preamp tubes inserted, but no power amp tubes.

MattPete · Post by **MattPete** » Thu Apr 05, 2007 10:13 am

novosibir wrote:You can't get correct readings and on some pins nothing to read w/o all the tubes in the amp. The tubes cause a voltage drop in the voltage supply rail and the tubes are connecting some pins in the sockets, where you now of course doesn't have any reading!

Larry

[now that I'm awake]

...so, what you're saying is that I should measure the preamp tubes with the power tubes installed, otherwise I'll get wacky values that won't match the voltage chart?

On the one hand, that makes sense (voltage drop, + current will be going from OT to NFB and to the pins with zero readings), but step 17 (or was it 16?) implied that I should measure the pres without the power tubes and compare those values to the voltage chart.

Hmmm...I'll try take some readings later tonight with all of the tubes installed.

thanks,
Matt

SDM · Post by **SDM** » Thu Apr 05, 2007 1:25 pm

You just need the preamp tubes installed to get good measurements here, their current draw does most of the voltage dropping through the B+ line. The output tubes only draw current right from the B+ start (through OT) or through the choke (screens). So having the output tubes in will drop the overall B+ a bit, but they are not needed to get the readings you need for testing at this point. Also if you haven't set the bias yet, you risk red plating them in the process.

So anyway get new measurements with just the pre's in, no blown/bad fuses, and also be sure your meter has good batteries and is functioning properly (no internal blown fuses itself). Set to DC volts for reading all preamp pins (except the heaters 4,5, and 9 which are AC of course). Amp will be fully on, not in standby.

MattPete · Post by **MattPete** » Thu Apr 05, 2007 6:02 pm

Ok, with the preamp tubes installed but with the power tubes out, I get these readings:

Power tubes (el34 - little variance between tubes)
1 0
2 2.9-3.0 VAC
3 477
4 475
5 -37
6 476
7 2.9-3.0 VAC
8 0

According to the charts, I should have a slightly larger drop between 3, 4, and 6 (should be 470, 466, & 460).

PreAmp:

V1-1 384
V1-2 0
V1-3 0
v1-4 2.9 VAC
v1-5 2.9 VAC
V1-6 388
V1-7 0
V1-8 0
v1-9 2.9 VAC

V2-2 391
V2-2 0
V2-3 0
v2-4 2.9 VAC
v2-5 2.9 VAC
V2-6 426
V2-7 388
V2-8 0.002
v1-9 2.9 VAC

V3-1 401
V3-2 0.001
V3-3 0
v3-4 2.9 VAC
v3-5 2.9 VAC
V3-6 394
V3-7 0.001
V3-8 0
v3-9 2.9 VAC

Other Readings:

Where the OT (white wire), choke, and fuse come together: 474 VDC

Presence to 8 ohm tap: 0

MattPete · Post by **MattPete** » Thu Apr 05, 2007 8:39 pm

Ack! For kicks, I installed the power tubes and turned on the standby. V6 and V7 glow, but the remaining tubes remain cold. Strangley, the readings on pins 2 and 7 are identical on all power tubes, as are 4,5 and 9 of the preamp tubes (all roughly 3 VAC).

MattPete · Post by **MattPete** » Thu Apr 05, 2007 10:52 pm

Well, I feel dumb. I reflowed a bunch of solder joints (meaning, just about everyone I could get my hands on). I also noticed that one of the 1k resistors was close to one of the heater pins on one of the output tubes. I couldn't tell if they were touching, but I bent them away just in case.

Now the preamp tubes are glowing, and all of the voltages are decently close to the ones in the voltage chart. Now to install the power tubes...

Yipee! The sweet glow of tubes! Next step: biasing, then installing the MV.

Moral to this story: When people suggest that you check lead dress, really check it. Don't just eyeball it: reflow the joint just in case.

....argh, I wired pin 8 to the ground lug, not pin 1. I'll need to fix that before I try to bias.