Tube swap on Hiwatt

[davoux]

It seems I should be worried about red plating new tubes when I replace the Mullards. I have both the plug in voltage and amperage testers. Can I put in only 1 tube and test amperage draw or do I need the full quad. That way if its too hot it only ruins 1 tube. Or, would it work to leave 3 of the Mullards in and replace just 1 while I test it? would that still give me the accurate current draw for that tube or would it change when you put the rest of the quad in?

Ray suggested I use Groove Tubes #1s and I may have to do that. However, my son tells me he has already ordered a quad of Electro Harmonix. I would hate to fry them all if they just won't work on the Hiwatt. TNX, John

[Ray Ivers]

John,

Here's what I would suggest:

1) Pull out the chassis, remove only the output tubes, then power on mains-only and use your DVM to check pin 5 of each output socket to see what negative bias voltage you have. My schematic indicates a non-adjustable -38 volts, but your amp may have adjustment(s) and/or a different reading. If there are bias adjustment(s), adjust for max negative volts on pin 5 of all output tubes. FWIW, the 200 watter I have has two (counter-rotating!) adjustment trimmers; they are both clearly stock, and wired differently from every Hiwatt schematic I've seen, 200W or otherwise.

2) Power off, install all the output tubes, and use your 'bias probe' (by this I mean a device that plugs in between the socket and the tube to measure cathode current - I'm not sure what you meant by "voltage and amperage testers") on one output tube.

3) Power on mains-only, let it warm up for maybe 15 - 30 seconds, then flip on the standby and check your current reading. If you get a reading of 40mA (74% plate dissipation) or less, you should be fine - 50mA (92% P.D.) or more, and I'd shut it down. Don't worry about the tubes during this brief test - they can handle more than 80mA for short periods with little or no risk of damage.

With a brand-new set of good tubes, your idle current should drop a bit after 5 to 10 hours of use, then stay stable for quite a while.

All the above assumes an "above all, do no harm" mindset , and a stock Hiwatt DR103 that matches the schematics I have, from this site:

http://www.mhuss.com/Hiwatt/tech2.html


Ray

[davoux]

Thanks Ray, You are right the DR103 has -38v bias circuit and mine is stock per the schematic - no trimmer. The bias tools I talked about are the kind you plug in, plug the tube into it, and it has 2 leads you plug into the meter. The one is to measure the B+ (big deal huh, they sent it by accident but wouldn't take it back) the other is to test current.

What I thought I would do is replace and read 1 tube at a time so if these tubes won't work I could potentially only sacrifice 1 out of the quad. What my question was getting at is if I would get false readings by changing only 1 tube at a time which probably would have very different conductance from the other 3 mullards that would still be installed when I do the 1st one. As I see it, the tubes are parallel to all circuits so all would have the same voltages. And, the current is derived by each tubes' individual conductance given that common voltage and bias. Not impacted by the conductance of the other tubes. So, if I read 36 to 40 ma ( I hope), that reading would stay the same as I change more tubes. Am I correct or am I missing something? TNX John

[Ray Ivers]

John,

the DR103 has -38v bias circuit and mine is stock per the schematic - no trimmer.

OK, great - that's a critical bit of info regarding what follows.

The bias tools I talked about are the kind you plug in, plug the tube into it, and it has 2 leads you plug into the meter. The one is to measure the B+ (big deal huh, they sent it by accident but wouldn't take it back) the other is to test current.

OK, that's good - knowing the B+ voltage allows you to calculate the plate dissipation, and of course the current reading is of prime importance.

What I thought I would do is replace and read 1 tube at a time so if these tubes won't work I could potentially only sacrifice 1 out of the quad.

Since you're getting -38 volts bias, IME nothing will get hurt - even if you get a high current reading, with this much bias voltage available it will never be so high that you you couldn't hit the standby switch long before tube damage occurred.

Also, just FWIW - I've had experiences where one power tube would affect its 'push-pull brother' - leading me to think both were bad - so I personally wouldn't trust any "3 old/1 new" testing, but YMMV.

Obviously, feel free to try different methods - mine was only a suggestion.

Ray

[Guitarist]

Somewhat related. I know the answer is "no, no good" but here's the Q.

If "Don't worry about the tubes during this brief test - they can handle more than 80mA for short periods with little or no risk of damage."

I was just working on a new Traynor and some FET area cicuitry went defunkt which caused me to bypass the regulation circuit that was bound up in there to just get it going for now BUT this disabled their fancy standyby scheme. So now it just ON or OFF. No redplating is happening, shes got big, normal volts again and the amp sounds superb (with a 12DW7 in V1).

Can I just label it as operational and encourage a yearly output tube change? If so what brand/type of tubes would tolerate this abuse?

My apologies Davoux if this is too off topic and happy NYear.

Edit: found this link: http://www.traynoramps.com/downloads...an/smycv40.pdf thanks Tubebass! Looks promising.

[Amp Kat]

Davoux, make sure you let the tubes stay on for about 20 minutes and burn in then check your bias and set it again if necessary. Do make sure you have a load on the speaker jack when doing this. As Ray said as long as the bias isn't at thermal runaway (basically zero) a tad of redplating won't hurt anything you just don't want to do it for a long time. I bet your very close to where it needs to be and you may have to just change the bias resistor a hair.

[davoux]

Too hot?

I replaced the tubes and had no red plating even when the son plugged in and played at about half volume.

The problem, they are running at 50ma which would translate to 22.5w or about 90% of 25w rating. How long do you guess these will last? Will there be any danger to other parts if they blow?

When you talk about tweaking the bias resistor, Which one is it and how much do I change it. The resistors I see in that circuit are: 1st, a 47k that parallels a 100uf directly after the diode. 2nd, is a 1k 1w directly after the trans. 3rd, there are (2) 100k one on either side where the bias connects and splits in the power tube network. 4th, there are 22k tied to each tube pin 5.

My son and I are both pretty ticked off. I had said the Mullards had begun to sound kind of tired. What really happened is that someone got into his amp and stole the full complement of Mullards. They left us a mixture of old junk. Then, to add insult to injury, they drilled a 3/8 hole right above the output jacks. Looks like someone was going to add another output to try to drive a third cabinet - that would have been a real disaster if they had taken out his partridge trans. He has been leaving it over at the house where the band practices. He won't do that again. He had saved his money and paid nearly $1,900 for it because it was pristine, now its not, so changing a bias resistor will not further reduce its value.

[Amp Kat]

When you first posted the Mullards were getting old that didn't jive with me as those tubes really don't get tired. They just go and go. Really sucks someone would do that but that's the way the world is now. Someone always out to screw the next guy to get ahead of the game no matter who it is. The resistor you want to change is the 47k paralleled with the 100uf cap. I'm showing -38 vdc of negative grid voltage is the number but that was back in 78 when the line voltage was about 120 and now it's 122 to 126 so that negative number may need to increase or more negative to say. Decreasing the resistance will make the negative grid voltage more negative decreasing the current thru the tube so if you took a 47k and paralleled it across the existing 47k it may set it just right. The goal is to get it around 70% and just out of crossover distortion but 90% or 50ma per tube is way to high. I'm thinking 36 ma's per tube sounds about right. I try to tackle that resistor change without taking the whole board off so whenever you get the right value you can just clip the resistor and use the existing leads to solder the new one in. I'm thinking around 33k will be the final value but that's just a guess.

[Tom Phillips]

The resistor you want to change is the 47k paralleled with the 100uf cap. I'm showing -38 vdc of negative grid voltage is the number but that was back in 78 when the line voltage was about 120 and now it's 122 to 126 so that negative number may need to increase or more negative to say. Decreasing the resistance will make the negative grid voltage more negative decreasing the current thru the tube so if you took a 47k and paralleled it across the existing 47k it may set it just right.

I’m just having my first cup of coffee but I believe you have that backwards. Here is my take:
The power supply should be similar to the one at

http://www.mhuss.com/Hiwatt/Schematics/DR_100wPS.pdf

1) The 1k and the 47k resistor act as a voltage divider. Decreasing the 47k to 23.5k by paralleling another 47k would make the bias volt less negative not more negative.
2) Since the other resistor in the circuit is already very low ( 1k) you are not going to be able to adjust the bias significantly more negative no mater what you do with the 47k. I estimate that even if you change the 47k to 100k the bias voltage will change more negative by only around half a volt. The circuit is almosed maxed out like a gain control that is already set to 9.8 out of 10.
I have often heard people talk about a voltage doubler mod in this situation. If I’m correct then someone here will have a link to further information.

Regards,
Tom

[Amp Kat]

That would make sense as more resistance is less current. I usually go one way and either series or parallel. Depends on the winding if it will allow enough or negative enough voltage. You may also be right that the voltage doubler is needed if that's the case. I may need some more coffee too Tom and take some breaks as I've been off a little here lately,

Hope your not confused but what were telling you is the winding for the negative voltage is not supplying a high enough voltage. You can try increasing the 47k to higher values but it's going to get to the point where it won't go any higher. heres a half wave voltage doubler circuit I found but unfortunately it's going to be hard to do this without pulling the board but is possible. The diodes will also have to be turned around and the caps as this is a positive one. http://www.tpub.com/neets/book7/27m.htm

[Robert M. Martinelli]

Cathode measurements

Hi Davoux,
maybe you' re not really at 50 mAmps, so there is a chance your actual plate current is less than that - If you' re measuring the current directly on the plates then your reading is good and you should di something to lower it, see the previous posts, these guys know what they' re taking about -

If you' re measuring the current across the cathode, be aware that the current you' re reading is the TOTAL current for that tube ( the sum of PLATE, or ANODE and SCREEN GRID currents ) and, since the screen grid current can go up to 8-10 mAmps, this means your plate current actually is about 40-42 mAmps and your tubes are perfectly safe.

To determine the screen grid current measure the voltage drop across the screen grid resistor and divide it by the screen grid resistor' s value - this is the current to subtract from the cathode current reading to get the actual plate, or anode, current.

I recently restored/repaired a HIWATT from the early 80s ( one of the latest models before the company ceased its activity ) and this had no bias adjustment as well, the tubes were running a little hot so I added a bias adjustment network ( well, actually two, one for each branch of the push-pull stage ) - but, as I told you, if your actual plate current is around 40 mAmps there' s no need to do so.

Hope this helps

Best regards

Bob

[Enzo]

And for goodness sake, stop worrying that off-bias will instantly destroy your tubes - it won't. People worry way too much about bias.

Bias is like tire pressure on your car. there is a recommended amount and you should stay close to it. But if you are off pressure a few pounds, the car will still drive you to town and back and won't explode. Obviously zero tire pressure can't be driven on, but even 15 pounds low, you can still drive the mile up to the service station. Just so with tube bias, it ought to be so and so, but it won't hurt them to start it up and see if there is trouble.

And yes, checking the bias voltage first is a fine idea.