Simple SE power transfomer question?

[ryc989]

Hi,

I am just putting together everything for my Simple SE project and had a general question about the tube types that can be used. I see that most tubes (EL34, 6L6GC, KT88, 6550, ect..) are all in the same general B+ voltage ratings, but if I wanted to use a 6V6 output tube, could I do that using the Allied 6K7VG?? If so, would it be as simple as using a different rectifier tube?? I don't want to overly complicate things as this is my first build, but I am also trying to understand options. I know that I would need a different value for the cathode resistor, and the value would be based on desired output power and B+ voltage. Any help and would be appreciated, also what would be a good B+ voltage in this amp for a 6V6 tube??

Regards

 

[Ty_Bower]

...if I wanted to use a 6V6 output tube, could I do that using the Allied 6K7VG?

No. You'll end up with too high B+ voltage. Use the Allied 6K56VG instead.

 

[rknize]

The 6V6 wants to see a higher primary impedance on the OPT as well, though it would probably do OK at 5k. Personally, I thought that trying to support the 6V6GT was not worth the trouble.

 

[Tubelab_com]

Any help and would be appreciated, also what would be a good B+ voltage in this amp for a 6V6 tube??

The 6V6 works best in triode with a B+ of 320 to 340 volts. This is a bit over the 315 volt spec, but you will lose a few volts in the cathode resistor and in the OPT. Many guitar amps ran the little 6V6 at plate voltages near 400 volts without exploding them, but this was in pentode mode. They will definitely not last long in triode or UL with the 6K7VG and even a dead rectifier tube.

I have a dedicated 6V6 Simple SE and it does use a 6K56VG, and Edcor XSE15-8-5K OPT's. The 6V6 does seem to work best with a 6K ohm load, but my speakers are well above 8 ohms for most of the audio range.

 

[ryc989]

Hi,

I really appreciate everyone's input, I am trying to learn as I go along. I think for now I will stick with the higher B+ tube types and perhaps build a second Simple SE amp designed for the 6V6 at a later date. I have already purchased the bigger transformer and I am looking at output transformer options. I have all the other parts necessary to do the build.
Regards

 

[ryc989]

Hi,

I have a question about the values for the cathode resistors on the output tubes. In general it seems like the 560 ohm value is a 'middle of the road' value, but if I wanted to use a larger value to lower the Diss and Ik, how would I know if the value were too large?? And if I used a larger value resistor, would impact the tubes I could use?? I plan on using 6L6GC tubes to start with, but I would like to try different type output tubes to see which ones I prefer.
Regards

 

[rknize]

The chart on this page is meant to help you answer that very question:

Tubes & Transformers

It depends mainly on the following factors:

1) Tube choice
2) Actual B+ voltage
3) Desired idle current of the tube (Diss)
4) Primary impedance of the OPT (higher turns ratio has more windings and therefore higher DCR)

There is also some variance with the tubes themselves. I would buy a couple of different sets of resistors so that you can change it if need be. 560 is a good first choice. I'd pick up a set of 470 and 680 ohms as well. Some of the Chinese 6L6 tubes seem to run hot and need a larger resistor.

Avoid bending the leads on the back of the board when you solder them in. It make it easier to lift them out later.

Once you make you choice, you can look at other tube types in the chart to see how they will operate. The orange Diss numbers indicate that they will run hot, but within spec (some tubes can't handle these levels even though they are within published specs). The red numbers are over spec, but some tubes handle this fine without glowing. The other numbers run the tubes colder. You will notice that distortion goes down and dampening factor goes up as the tube is run hotter (last three columns). That is the trade off of tube life versus performance.

 

[nic6paul]

There is also some variance with the tubes themselves. I would buy a couple of different sets of resistors so that you can change it if need be. 560 is a good first choice. I'd pick up a set of 470 and 680 ohms as well. Some of the Chinese 6L6 tubes seem to run hot and need a larger resistor.

The coke bottles from AES sure do run hot! The EH 6l6gcs have no issues running with a 560ohm resistor in.

 

[Ty_Bower]

In general it seems like the 560 ohm value is a 'middle of the road' value, but if I wanted to use a larger value to lower the Diss and Ik, how would I know if the value were too large?

I started with 560 ohm resistors, but decided my tubes were running too hot. I added another 250 ohms in series with the original 560 ohm resistors for a total of 810 ohms. This seems about right for most EL34/6L6GC type tubes.

The 560 ohm resistors were good for quality KT88 tubes. I can easily go back to 560 ohms just by putting a small shorting jumper around the extra 250 ohm resistor.

http://i69.photobucket.com/albums/i43/Ty_Bower/Simple SE/P1100584.jpg

 

[rknize]

Or you can get all fancy-pants:

 

[Ty_Bower]

Geez, Russ, you do nice work!

 

[rknize]

Thanks Ty! It's more about being really anal. I put binding posts there, but I knew full well that I would probably almost never swap the resistor because it means flipping the amp over and opening it up. I'm just too lazy, LOL...

 

[ryc989]

My Board Pictures

Hi,

I just wanted to post a few pictures of my board, I ordered my output iron and will have to wait 4 to 6 weeks for it to arrive. I might order a set of small Edcor output transformers so I can test my setup and order my simple pp power transformer at the same time since that is my next project.

Regards

 

[rknize]

Looks like it's ready to rock.

 

[ryc989]

My Simple PP Board

Hi,

Since I am waiting for iron for both my projects I decided to build my simple PP board. I have attached pictures, the board is very heavy once finished, btw, ! I guess I will move onto building the chassis for both my simple SE and simple PP projects and will attach pictures as I make further progress. One note, the space for the coupling caps got very tight. I think that some thought needs to go into the size of caps being chosen, I used caps that I feel will give me the sonic results I want, i.e. transparency and little sonic signature. I had some challenges getting them in place and keeping the leads away from several of the tube socket pads. Just a heads up to other people who will be building this project.

Regards

 

[ryc989]

Amp check out

Hi,

I was doing the checkout on my amp and after a Kodak moment and realizing that I was using AC instead of DC to check the voltages everything appears to be ok?? I just have a few questions:

If I ohm out the primary side of my output transformers should I see a 5K resistance?
I am measuring about 380 ohms and the UL tap is about 160 ohms. Also, when measuring across the output of the transformer I only see a little over 1 ohm??

The reason for asking is when checking out the amp with audio in, I was able to get sound and the volume level seemed ok, just the sound was a little muffled?? I am also not sure that the tubes were not running some what hot?? I am using a 1k resistor for R17 and R27. I am just wondering if there isn't some issue with the output transformers?? I tried both 6L6 and EL34 tubes, although the EL34 tubes seemed to glow overly bright , so I just shut the amp down.

Regards

 

[rknize]

If I ohm out the primary side of my output transformers should I see a 5K resistance?
I am measuring about 380 ohms and the UL tap is about 160 ohms. Also, when measuring across the output of the transformer I only see a little over 1 ohm??

That sounds about right. 5k is the primary's AC impedance, which is not the same thing as the DC resistance (what you are measuring with your meter). If you want to know more about AC impedance and why it ends up having units of ohms, there are lots of sources on the Internet and books.

The reason for asking is when checking out the amp with audio in, I was able to get sound and the volume level seemed ok, just the sound was a little muffled?? I am also not sure that the tubes were not running some what hot?? I am using a 1k resistor for R17 and R27. I am just wondering if there isn't some issue with the output transformers?? I tried both 6L6 and EL34 tubes, although the EL34 tubes seemed to glow overly bright , so I just shut the amp down.

What was glowing? The plates or the filaments? 1k is a little soft for a cathode resistor and might explain your weak output. Next time you power it up, take some voltage measurements.

1) B+ This is the voltage (from ground) at R1, R4, C2, R14, R24, and the red wire of your OPT.

2) Across R17 or R27 (one meter probe on each leg of the resistor). This is the output tube bias voltage.

3) Across R13 or R23. From this we can measure the input/driver tube plate current.

Also, some pictures of your wiring might be helpful.

 

[ryc989]

Listening to it!

Hi,

Thank you for the reply so here are the voltages I measured(all DC measurements):
no R1 have choke but 487v
R4 487v
C2 487v
on one side of R14 400V - 397V fluctuates a little the other side 487v
on one side of R24 398V - 396V fluctuates a little the other side 487v
across R17 45v
across R27 45v
across R13 2.88v
across R23 2.88v

I picked the value of 1K based on the output transformers I will eventually use, rated for 60ma. I have purchased a set of transcendar opt's to use to test this amp, they are rated for 90ma. The 45v is what I was expecting or looking for I believe, 45v/1000ohms = .045a = 45ma I have re-checked everything and I am using a better pair of speakers now. I am using the diodes as a rectifier as the only 5AR4 tube I have currently is way to valuable to risk. I have ordered some new 5AR4 tubes and will use them once they arrive. The amp actually plays more than loud enough for me. I am using a preamp and my only source is vinyl and my first impressions now are the sound is good(bass is different!). I am using $4 Chinese 6L6 tubes (eBay special,I am going to try my $4 EL34's next!) so I am not sure how good the sound will or will not be but I just want to make sure everything works for now. I know the pictures look like they came out of a 50's 'B" horror movie, but I am going to reuse this setup for my simple pp amp once this project is complete. I really do appreciate all the help and support here, I do not think I could find a better place or group of people to build a project with!

Regards

 

[rknize]

Everything looks good based on your setup. Your B+ is fairly high right now...a combination of the Hammond and using the SS diodes. With the 5AR4 in there, the tube drop plus the extra load from the 5V heater will bring it down some. The 6L6 is running in a region that is not as linear as it could be if it were run with a lower Vgk. Take a look at the triode curves for the 6L6GC and you will see what I mean. I would pick up a couple of other sets of cathode resistors if you haven't already. They are cheap and it makes it worth experimenting.

 

[Ty_Bower]

Your voltages look perfect. Each half of the 12AT7 is drawing 8.7mA, which is where it should be. The power tubes, drawing 45mA each, are dissipating a total of 19.9 watts. I'd figure that's pretty much ideal. The Transcendar output transformers are pretty good performers. I have a set of his 5K:8 in my Simple SE and I'm extremely pleased with them.