Tubelab Simple SE low B+

[mpri]

Hi all.

I have just completed a build of the Simple SE. After doing some initial resistance tests I put tubes in to do voltage tests and the B+ seems low for what I was expecting.

Here are my PS components using a CLC filter:
Allied 6K7VG 750VCT
35mfd motor run cap at C1
Triad C-14X 6H 150 ohm choke
100mfd Electrolytic cap at C2
50mfd motor run cap optional auxiliary cap for C2
5U4GB rectifier tube
CL-90 inrush current limiter on main fuse/switch side
CL-90 inrush current limiter on PT secondary CT (red/Yel)
Wall voltage is 118V

With this my B+ measures 391V. PSUD II shows it should be around 435V. Looks like my system is about 40V less than it could be. I know I can use a 5AR4 rectifier and get about 30V more. However I have installed a bank of switched resistors for tube rolling based on an expected B+ of 450V– 460V with a 5AR4 and I’m thinking that will only get me to about 425V or so.

The amp does work. Just sounds a little underwhelming as the cathode bias resistors are wrong for the B+. I did try another 5U4GB, but it didn’t change much. My questions are does this voltage seem reasonable? Is there something I could do in the PS filter to get more out of it? How do others get 450V?

It seems with this setup most people get about 450-460v B+, so that's what I was expecting. When I didn't get it is when I modeled it to see what may have gone wrong. The one difference I have from others is I used a motor run cap for C1 since 500V 105 degree electrolytic caps are hard to find now. Also in PSUD C1 is an electrolytic. Mine is a motor run cap. Maybe there is a difference?

Any info or suggestions would be much appreciated.



Thanks,
- Mike

 

[mpri]

Here’s some other info on measurements I made tonight.

With the 5U4GB in I measured voltage at C1 and it was 411v. I replaced the GE 5U4GB with a Sylvania 5AR4. Measured voltage at C1 and it was 435v. B+ seems to fluctuate between 416-425v with the 5AR4. C1 is a motor run cap I picked up off eBay. Suppose to be new, Ackard 35MFD 440VAC, seems fairly standard. Maybe it’s not working too well.

Any ways, like I said the amp is working, just not as I expected with the lower voltage. Here are some measurements I took tonight. OPT’s are Transcendar 5k’s. B+ was about 418v at the time with the 5AR4.

Chinese 6L6GC
635 ohm cathode resistor. 33.7v across R17 and 401v on the plate = 33.7/635 * (401-33.7) = 19.5 watts.
560 ohm cathode resistor. 32.1v across R17 and 394v on the plate = 32.1/560 * (394-32.1) = 20.6 watts.
470 ohm cathode resistor. 30.2v across R17 and 387v on the plate = 30.2/470 * (387-30.2) = 23 watts.

EH EL34
635 ohm cathode resistor. 32.5v across R17 and 402v on the plate = 32.5/635 * (402-32.5) = 18.8 watts.
560 ohm cathode resistor. 31v across R17 and 396v on the plate = 31/560 * (396-31) = 20 watts.
470 ohm cathode resistor. 29v across R17 and 385.5v on the plate = 29/470 * (385.5-29) = 22 watts.

So everything is running, just on the low side. Looks like I won’t be able to run KT88’s unless I use a cathode resistor of like 200 ohms!

Does this look normal? Seems like from other posts some have higher voltage and use larger cathode caps. Would replacing C1 be a place to start?

Thanks,
- Mike

 

[rknize]

What VAC do your measure coming off of the secondary of the power transformer? You can also try bypassing the CL90 on the secondary. It's not really enough current to get it hot. That will account for some of the drop. A CL140 works better here.

 

[mpri]

Oh yes, I should have mentioned the secondary voltage. At the board across the secondary input connectors I measured 376VAC. Seems spot on. And yes I will try and remove the CL90 from the center tap and see what difference that makes. Maybe others who get more voltage with this setup don’t use the center tap CL90, or use a CL140? I put it in to reduce the surge on the rectifier during warm up, but if it lowers the voltage by what I’m seeing, maybe I’ll do without it.

Thanks,
- Mike

 

[rknize]

It's hard to predict what the CL90's resistance will be, since that is outside of the spec'd range. I would expect a drop of 5-10V or so.

376VAC is right on...which is quite unusual for the Allied transformer. This may account for what you are seeing. The cheap Allied transformers (made by Hammond), have a reputation for running "hot". That is, all of the secondaries output higher voltages than they should. They get toasty as well. It's believed that they are really designed for 110V operation and therefore run hot on today's 120V power lines. Maybe they have improved their design or maybe you have lower line voltages than most or maybe you got "lucky" and got one that doesn't run so hot.

What do you measure coming out of your wall outlet? How do the other secondaries measure?

 

[rknize]

I should add that I have an Edcor power transformer that comes in right around where yours does. I see on the order of 420-450V B+, depending how much current I am drawing. So if you account for 10V or so from the CL90, that sounds about right. I can select a cathode resistance from 340 - 560 ohms, which works well for anything from EL34s to KT88s.

 

[mpri]

Hmmm, not sure why I wrote 376V in the last post as that's only half of it, right? I must have measured the 376v somewhere else. These are my measurements taken the other night from my notes:

Wall plug: 118VAC
Red wires – secondary: 757V (half is close to what I said earlier, 378.5V)
Yellow wires – 5V: 4.92V
Green wires – 6.3V: 6.41V

Also at the output of the rectifier tube, C1 plus side, with the 5U4GB I measured anywhere from 407V – 411V with a B+ of 388V – 395V. With the 5AR4 I measured at C1 from 434 - 438V with B+ of 416V – 425V.

Thanks,
- Mike

 

[mpri]

Russ – From the forum postings I’ve read it looked like more people were getting a B+ around 450V and using higher bias resistors with the same components. That’s partially why I chose them as I thought I knew what to expect and designed my bias resistor switch range accordingly. Looks like you’re running not much higher B+ than mine with similar components, and accordingly your resistor range is on the lower side also. Right now my bias range is set at 470, 560, 635, 741 and 820. As of now, running either the EL34 or 6L6GC I’m using the 470 ohm bias which gives me about 22W – 23W dissipation, which is fine. Just leaves me no headroom for a KT88. Looks like I need to remove the higher end resistors and add some to lower end.

Ok, I guess all is good then. Thanks for working this through with me.

Regards,
- Mike

 

[rknize]

If you measure from the center tap, you'll get exactly half. That is probably what you did. Anyway, it sounds like you have a rare case of an Allied transformer that actually works the way it is supposed to. All of your secondaries are spot-on. Indeed, I also would have expected that you would get more B+ with that transformer.

A little less B+ shouldn't make much difference in the performance of the amp, once you get the cathode resistors sorted out. In fact, you should actually get a bit less distortion since you can run them with more plate current than you could have with higher plate voltage.

 

[mpri]

Russ,

Yeah, center tap, that’s where I got it. I decided to leave the CL90 in for now. I can always pull it later if I decide I need the extra voltage. I only have a pair of EL34’s and some 6L6GC’s, no KT88’s yet, but I plan to purchase some down the line. The amp is still in a temp prototype base. I’m in the process of making a proper base for it, so when I move it I’ll swap a couple of the higher bias resistors for some lower ones to accommodate KT88’s.

I’ve dabbled a bit in electronics before on a smaller level. Fixed some old 30’s and 40’s AM radios, built a one tube AM transmitter, but nothing like this. I’ve been reading the forums and planning this out for awhile and thought I knew what to expect for voltages and everything else so when the voltages were off I thought maybe I goofed something up or had a bad part. However I guess most non high-end parts are + or – 10% so results may vary. Any ways right now I’m running an old set of RCA black plate 6L6GC (thanks’ Wurlitzer) with a 470 ohm cathode bias resistor at 23W and listening to some good jazz. It does sound real nice and nothings running hot. I’m happy.

Thanks Again,
- Mike