HT Stage 60 Valve & Speaker upgrade

[Maxsys]

Thanks for that, exactly the answer I was looking for, I can stop worrying now, and yes it sounds awesome!!

[hepsen]

Hi Maxsys!

I'm glad that there is a current thread concerning the exact amp & similar upgrade plans that I do have.
I bought the Stage 60 used some weeks ago. The amp is around 5yrs old and the original TAD tubes and Celestion Seventy 80 speakers were not my kind of sound. So I'm thinking about a tube change (V1: Tung-Sol ECC83 Ri V1-selected, V2: Tung-Sol ECC83 Ri, V3/PI: either JAN 6189W GE or Tung-Sol 6189 Ri, PT's: either JJ EL34L or Tung-Sol EL34B Ri). With the speaker combination I'm not done deciding yet. It's more like a weight against sound choice right now (G12H-30/V30 combination vs. a duett of Celestion's Neodyme Series).

I do like to bias the amp also by myself. I appreciate the pictures you've posted here. I was asking in other forums for help/advice how to do this step by step. Most people weren't very helpful and just ended the threads with something like "go to an amptech". YES, I KNOW that there a Voltages inside the amp that might kill me. But I also know that KNOWLEDGE will keep me safe working inside an amp. And because I like to live on I will also made my decision on the knowledge I can get wether I do it by myself or will let it be done by an amptech.

• 1. The first thing to me is the safety side and by that the question on how and where to unlead the high voltage capacitors (with also measuring the voltage dropping)? I do not want to place the DMM anywhere on the board before I'm sure it's safe!

• 2. Measuring the plate voltage: you measured between pin3 and ground (chassis)? In a handbook I found hints to measure from top inside of the socket (no tube installed) between pin8 to pin3. Which lead should be on which pin in this case (pin8: black/pin3: red)?

• 3. To measure current I'd like to use a socket adapter. That adapter measures a voltage that is equal to the current of the cathode. Would you prefer measuring both sockets at the same time? I guess there might be small differences between the tubes and I would prefer to adjust the hottest of both to around 65-68% by using the PR2 pot. So to me right now it's more a matter of price because I'd need another DMM and a second socket adapter.

• 4. Have you done adjustments on the PR1 pot to adjust hum?

Best regards
Heiko

[Maxsys]

Hi There Heiko,

In response to your specific questions.

1. I have no idea how to do this, and too be honest i don't think it will make any difference, the amp needs to be powered on to take any measurements, so the caps will be charged. If you are uncomfortable with this then the stock answer applies, take it too a tech.

2. Again no idea, and if there is no load from the tube on the socket i don't understand how you would measure anything? I saw nothing in any handbook about this method.

3. If you are using an adapter, then you will need to follow the manufacturers instructions for that adapter. I have never used one i guess it is a safer alternative as you are not prodding around with a meter yourself. If you buy matched tubes from a reputable source you should have no issues with having one hotter than the other, mine are running at identical voltages.

4. I don't understand what you mean about hum, one pot is for balancing the two tubes, one is the bias adjust, i never noticed any hum on mine either before or after. Perhaps your tubes do need changing and this is causing the hum.

Please understand i am no expert just a keen guitarist who likes to take care of my own gear, i only resolved this myself with the help of others.

I am not sure if that has helped you but those are the honest answers.

[thephantum]

• 1. The first thing to me is the safety side and by that the question on how and where to unlead the high voltage capacitors (with also measuring the voltage dropping)? I do not want to place the DMM anywhere on the board before I'm sure it's safe!

• Like Maxsys said, the amp must be powered on to measure and adjust bias, so there is no point in draining the filter caps. Having said that, if you plan to work on the amp with it powered down, the easiest way to drain them is to leave standby off (like when you are playing) when you power down the amp. All voltage in the filter caps should drain into the tubes. You can verify that they are drained using a voltmeter.
  
  If not, the procedure is to use a jumper to connect the positive lead of the cap to ground (chassis) on the amp. The problem is that it will spark. So add a 1W resistor to the jumper, 470 ohms or higher. This will slow down the discharge and prevent it from arcing. Of course this procedure would have to be done on all the filter caps to render the amp safe.

The biggest safety tip I can give you is to keep your left hand behind your back and only probe using your right hand. This is important, even if you are left handed. That way, if you do get bitten, it goes down the right side of your body to your feet. Yea, it'll hurt like hell, but most importantly, it doesn't go across your heart....which is what will kill you.

• 2. Measuring the plate voltage: you measured between pin3 and ground (chassis)? In a handbook I found hints to measure from top inside of the socket (no tube installed) between pin8 to pin3. Which lead should be on which pin in this case (pin8: black/pin3: red)?

• To measure plate voltage, the tube should be installed and the amp must be powered on. You can measure between pin 3 and the chassis, as that will give you the Plate voltage in reference to chassis ground. However, that's not the voltage that the tube is "seeing", as the tube is not directly grounded to the chassis. The Cathode (pin 8) is ground to the tube. If you measure between pins 3 & 8 without a tube, you will get a voltage, but, again, it won't be what the tube is seeing, because it's not in the circuit as a load.
  
  To get that, you need to have the tube installed and measure between pin 3 (Plate) and pin 8 (Cathode). The voltage differential there is the actual plate voltage that the tube, as a load, sees. Red lead should always be on pin 3...but if not, your meter will just read the same voltage as a negative value.

• 3. To measure current I'd like to use a socket adapter. That adapter measures a voltage that is equal to the current of the cathode. Would you prefer measuring both sockets at the same time? I guess there might be small differences between the tubes and I would prefer to adjust the hottest of both to around 65-68% by using the PR2 pot. So to me right now it's more a matter of price because I'd need another DMM and a second socket adapter.

• A socket adapter is the safest method for measuring. Even a halfway decent one will let you read plate voltage and bias current (reading bias current as a voltage is just measuring it across a 1 ohm resistor....Ohm's Law). Personally, I recommend two adapters. Yes, you'll need a second DIMM as well but it's much easier (and quicker) to dial in your bias when you measure both sides of a push-pull pair simultaneously. The tolerances on matched pairs of tubes are usually within 5%. So while there can be some variances, it's not enough to cause any issues...but it's always worth checking.

• 4. Have you done adjustments on the PR1 pot to adjust hum?

• PR1 is used to adjust the bias balance across the pair of tubes (PR2 is used to adjust the overall bias level). It's true that if a push pull pair is very far out of balance it can cause some hum, but PR1 is not specifically to adjust hum. It's to make sure that both tubes in the pair are operating at voltage/current levels that are as close to identical as possible.

As I alluded to in the answer to your third question, you can't adjust balance unless you are measuring both sides of the pair. That's a pain in the neck with one probe/meter...which is why I recommend using two.

The basic procedure is to measure plate voltage of each tube first, then adjust balance, then adjust bias, then re-check your plate voltages and repeat. As you adjust bias, your plate voltages will change, so you have to go back and forth until it's dialed in. This is another reason why I said you shouldn't measure plate voltage without a tube installed (as you asked in your second question). Yes you'll get a voltage, but it's not in respect to the specific tube being used. So there will be no way to dial in your bias based on the plate voltage of that tube.

Ideally (and this it how I do it) use two socket adapters and four DIMM's. The adapters only let you take one measurement at a time per DIMM. So you have to measure plate, switch leads, measure bias, and go back and forth. By using four DIMM's, I can measure both plate voltage and bias current for two tubes at the same time. *POOF* and I'm done...so I get back to playing the amp quicker.

Hope all that helps...

[hepsen]

Hello phantum,

thanks for your advices!

I've done the tube change and re-biasing recently by myself and I'm still alive!!!

What I've used:
Two DMM's, two bias probe adapters, safety leads with crocodile clamps, a safety philips screwdriver, lots of amp tech literature and forum research, Ohm's law and a calculator.

How I've done it:
[THIS IS NO HOW-TO-GUIDE OR ANY OFFICIAL ADVICE!!! THIS IS JUST A DESCRIPTION ABOUT HOW I'VE DONE IT AND WHAT HAPPENED. I DO NOT GUARANTEE FOR THIS AS BEING SAFETY OR WILL TAKE ANY RESPONSIBILITY FOR THIS!]
- After last playing I turned it off directly (without activating Standby) to drain caps fast. When the tubes were cold I removed every cables and the chassis.
- On my cleaned up workbench I first checked voltage between pin 3 (red lead) and the chassis (black lead) with the DMM (auto range VDC) - it was already around 0-1 V. I removed all tubes and put the new ones in.
- I reconnected one speaker, a cable to the input (to deactivate bypass), connected the leads of the DMM again to pin 3 and chassis, connected the power cable, checked EVERY connection twice to be secure and only then turned power on. The voltage went to 529VDC.
- After some minutes of warming up (and checking that the tubes for non glowing plates) I deactivated Standby and the Voltage dropped to around 508-510VDC (later I used this value to calculate the 50% and 70% values of the tubes).
- Again I directly turned of power and watched the caps draining on the DMM. It took a while until it was again between 0-1V and I removed the power cable for security.
- With the measured value of the plate voltage I calculated 50% (25Watt x 0,5 / 510VDC) and 70% (25Watt x 0,7 / 510VDC) as my biasing range an decided to go a little more to the hotter side around 64-66% what would be around 31-32mA.
- After the tubes were cold again I installed the bias probes for the power tubes, connected them to the DMM's and removed the leads from pin3 and the chassis before I reconnected the power cable.
- I switched the DMM's again to VDC (autorange in my case) and turned power on. In this case there is no voltage in standby where the bias probe measures (OF COURSE THERE WILL BE HIGH VOLTAGE IN THE CIRCUIT!!!).
- After deactivating Standby I've measured Voltages around 30mV (what is equal to mA because of the circuit of the used bias probes) on both sides. In the first minutes the values fluctuate a lot around a half mV/mA until it gets more stable. First I lowered the value a bit to see what happens over time and visibly checked the power tubes from time to time.
- Then I began to higher the value in small steps. This is not as easy as the bias pot range is VERY sensitive!!! When values get stable I've also adjusted the balance pot slightly. After every change I checked also the sound and humming by dialing in Volume and Tone controls of channel 1. I ended up at 32mV/mA after a while and let it settle over about 30 minutes.
- Again I turned the power off directly, removed the power cable and waited some minutes before I checked voltage between pin3 and chassis again to be safe before removing the bias probes.
- I reinstalled the chassis into the amp housing and that's it...

Have I forgotten something or calculated wrong?

I'm very satisfied with the sound right now!

One thing I've recognized afterwards (or maybe never checked before): the amp housing is warming up very high in the area where OD1/OD2 EQ is (especially the chassis screw there). It's not in a "melting" area but still very very warm. Do you recognize this on your amps too?