what could cause this?

krash · Post by **krash** » Tue 05/02/06 10:36 am

Open-circuit (no tubes installed), B+ is 407V. I have 150 ohm cathode resistor.

With tubes installed, the cathode voltage rises and if I leave the amp powered up, it will eventually rise to nearly 40V and then stabilize between 35 and 40V. Once it goes much past 10V B+ also starts to drop rather dramatically to like 275V.

Are there any causes for this behavior other than bad tubes? I think due to a mistake in dropping resistor, I may have killed my tubes on the first power-up of the amp. What else should I check? I have been over the power amp ckt with a fine-tooth comb and everything seems to check out.

Roudyrockr · Post by **Roudyrockr** » Tue 05/02/06 11:32 am

Krash,
the cathode voltage on which tube(s)? The power tubes? As the tubes warm up and the current starts flowing through your tubes it is expected to see the cathode voltage go up in reference to ground. This is the nature of cathode biasing, but, 40 volts on a standard 18 watt is high and would indicate higher then normal current flow.

It's normal to have a high B+ without tubes ( I assume you have a rectifier tube in or have a ss rectifier) since there is no current draw. It's also normal for the B+ to go down when tubes are inserted. If you're working on an 18 watt and you're measuring B+ at the standby switch, 275 is low. Low B+ like that is usually a bad rectifier that can't deliver under load or an excessive current flow somewhere dropping the voltages in the amp.

So, it would seem that you have excessive current flow somewhere. I would use the site's voltage charts and compare the cathode voltages on all tubes. It might just be a single bad tube. Also try putting in one tube at a time to see if you can isolate the section dropping the voltage.

krash · Post by **krash** » Tue 05/02/06 12:47 pm

Yes the cathode of the power tubes. This is a 36W so there are 4 power tubes and SS rect.

This all started when I first fired it up and had made an omission of the 1K-1.5K dropping resistor (between sections of the 32/32 cap on the current schemo), and that along with 125 ohm cathode res, I was getting redplating in the power tubes. Before I found my error, I had tried 10(!) different tubes in the amp and now whichever quad I put in there has this high cathode, low B+ problem.

I figure I just probably killed 10 tubes by running the screens way too hot having omitted the dropping resistor. Before trying other tubes in there I wanted to be very sure if there was ANYTHING else that could cause this condition.

krash · Post by **krash** » Tue 05/02/06 1:37 pm

here's a Q ... did I likely melt the screens on all 10 of these tubes? Of so can I test them and find out?

Roudyrockr · Post by **Roudyrockr** » Tue 05/02/06 2:43 pm

Krash,
I doubt that, but anything is possible I guess. You're only talking about a 10 volt difference from the plate voltage and the screen voltage supplies. You basically had 332 VDC on the screens instead of 322VDC minus the drop acrossed the board screen resistors. Do you have access to a tube tester? What was the initial symptom on start up that caused you to do all of the tube swapping?

krash · Post by **krash** » Tue 05/02/06 3:07 pm

It's not so much the screen voltage I would be concerned about, but current. However, I don't know nearly enough about this to be able to tell if this is a problem.

I discussed this on the 36W forum but thought someone on this forum might have more info. This is a rather atypical build but the symptom was the first time I fired the amp up, it worked but didn't get very loud and the tubes (very well-used Sovteks with years of hard miles) were mildly redplating when they got warm, even when idle, and the sound was cutting out. Eventually I discovered one tube had a crack in the glass and figured that was the problem (no measured voltages yet). Then I put in another pair of Sovteks in place of the damaged pair for continued testing and got it to play again, but with redplating and cutting out, until it stopped making any sound at all. I measured voltages, found the voltages way off (cathodes 35V, plate down to 274V), so figured I had hosed these dying tubes and put in a more robust (used, Valvo) set. Same effect with the other set but with the Valvo tubes, the cathodes go to 38V but still no sound, some redplating while idle, plate voltage drops to way below 300.

Since then I have pulled every component, changed the cathode res. to 150 ohm (was orig. 125), added in the 1K res that I had left out, and measured, checked, double and triple checked everything that I can think of. I also have one other good (maybe) tube to replace the cracked tube, but between these 10 tubes, any combination of tubes produces the same result.

So anyway, I figure I've probably got the problem sorted out but before risking another set of tubes, I want to check everything I can without tubes in the amp. So I was just trying to gather information about what COULD cause this, so maybe I'd know what to check.

Roudyrockr · Post by **Roudyrockr** » Tue 05/02/06 3:38 pm

krash wrote:So I was just trying to gather information about what COULD cause this, so maybe I'd know what to check.

Krash,
I did a search and found your other post on the amp. I see that you have a diode rectifier and several other changes from the Richie layout. Do you think you could mod the original layout with white out or something and scan it or take some pics of your chassis interior? Did you check the diodes to make sure one of them didn't go south?

krash · Post by **krash** » Tue 05/02/06 4:00 pm

well i'm pretty sure the diodes are ok since i get the correct DC voltage on the plates etc. until the tubes warm up.

but yeah, there are pictures. look here:

http://prophetsandpoets.com/josh/images ... ready_.jpg

here's a good close-up missing one component now in there (the cathode res. in the PI):
http://prophetsandpoets.com/josh/images ... stack_.jpg

in that picture, you can also see my cathode res (now 150K, same type resistor).

I guess I'll go double-check those diodes.

Roudyrockr · Post by **Roudyrockr** » Tue 05/02/06 4:54 pm

I don't see the 2.2k 2 watt dropping resistor for V1 and V2 on the board is it somewhere else? Is that it sitting sideways in front of the filter cap. I did notice the missing cathode resistor for V3. Where is the NFB switch. I don't see that.

krash · Post by **krash** » Tue 05/02/06 5:07 pm

Roudyrockr wrote:I don't see the 2.2k 2 watt dropping resistor for V1 and V2 on the board is it somewhere else? Is that it sitting sideways in front of the filter cap.

yes. it's more clear in this picture:

http://prophetsandpoets.com/josh/images ... al_ch_.jpg

I did notice the missing cathode resistor for V3.

I hadn't installed it yet when I took that picture. it now has a 1K in there (waiting on the 820 from Mouser, ~1wk)

Where is the NFB switch. I don't see that.

No neg. feedback.

the input to the PI is done like a regular 18W instead of the richie style. the "Normal" channel in this amp is like the vib channel from a regular 18W but with no vibrato, and the vol/tone wiper connects to the 0.1uF side of the PI input. the TMB channel MV pot wiper connects to the .022uF side of the PI input.

I guess you could say this amp is roughly based on Richie's 36W design with a small number of (mostly pre-PI) changes.

krash · Post by **krash** » Tue 05/02/06 5:19 pm

OK I know I really don't know what I'm talking about, but it seems that the way I had it connected, I essentially had the amp triode-wired for the outputs (screen almost attached to plate ... 100 ohms res.). If I got 35V across the 150 ohm cathode resistor then I was getting about 120mA/tube plate current at zero signal, which is FIVE TIMES the max 24mA/tube rating for an EL84 with triode connection.

Would this not likely smoke the tubes?

Roudyrockr · Post by **Roudyrockr** » Tue 05/02/06 5:20 pm

is there any way you can document the actual circuit of your amp like I mentioned before with a marked up schem or layout? It's difficult to make recommendations not knowing exactly how this frankenamp is put together.

Roudyrockr · Post by **Roudyrockr** » Tue 05/02/06 5:31 pm

krash wrote:OK I know I really don't know what I'm talking about, but it seems that the way I had it connected, I essentially had the amp triode-wired for the outputs (screen almost attached to plate ... 100 ohms res.). If I got 35V across the 150 ohm cathode resistor then I was getting about 120mA/tube plate current at zero signal, which is FIVE TIMES the max 24mA/tube rating for an EL84 with triode connection.

Would this not likely smoke the tubes?

based on ohms law 35v/150ohms would be 250 milliamps. That would be assuming that the grounded lead of the resistor is definiately at zero volts and there is no miswire. I'm sure you were using chassis ground rather then the resistor's grounded lead for you meter reference right? There is definitely something not wired right in this amp.

steveh · Post by **steveh** » Wed 05/03/06 1:06 am

This might be a really stupid suggestion, but is the load side of the OT (speaker jack) all wired up properly - as wouldn't you see this sort of o/p stage runaway if there was no load connected? Just a thought!

Steve

krash · Post by **krash** » Wed 05/03/06 9:00 am

I'll re-check the load-side of the OT. That's a very good idea.

Frankenamp! Ha. I like that. That's about appropriate. It's not exactly as you imply, though, other than the fact that it's built using salvage iron and chassis. 36W 18W-style amp on the whole is a "frankenamp" as the TMB a "frankenamp". Aside from the SS rectifier, the whole power supply and power stage of this amp is taken nearly 100% verbatim from Richie's 36W schemo and layout.

I guess I could try and whip up a marked-up layout or schematic. Schematic doesn't do any good though since obviously either I have killed off some tubes or I have miswired it according to my intended schematic.

krash · Post by **krash** » Fri 05/05/06 9:47 am

I FIXED IT!!!

woo-hoo. it works now!

I had failed to connect ground to the 470K grid leak resistors ahead of the power tubes.

What made this hard to find is that when it is powered up, this point measured 0V to ground even though it was not connected to ground (false ground resistor divider). However it failed the no-power continuity check and I found the error when I pulled the board and looked at the back side.

Now most of the voltages check out correctly except one ... maybe a bad preamp tube... plate voltage 327V, screens 325V (looks high to me), and V3 pins 2 & 7 are about 10V lower than I expected they would be (about 33V).

Thanks for all of the input ...

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