Amp Build From a Vintage Radio

[PRR]

The DC on the cap will be up-to 1.4/0.9 times higher, 578V.

With a couple 6V6 sucking 80mA-100mA through a true 5Y3 it will be 430V-410V DC.

[blown240]

Is that good?

Sorry....

[PRR]

Plan a place for a HOT dropping resistor to get down in the 300s.

There's a 1K slide-tap back-bias resistor on the original which might be the cat's meow.

Unless you are gigging-out, a couple 6V6 shouldn't need more than 300V 350V or so. In cathode-bias with reasonable OT impedance, you really don't want more than this, tubes melt or clean power drops. (400V works for fixed-bias max-scream stage amps.)

[blown240]

I am planning on running the choke that was in the original radio.  Hopefully it will drop the voltage some, if not then your suggestion sounds great.  Thanks again for all your help and info!

[sluckey]

According to PRR's schematic, there was a 1140 ohm field coil in series with a 7750 ohm resistor placed directly across the B+ line. That's a pretty hefty load and will probably knock the B+ down to  a managable level. You could use the original 7750 resistor plus a 1K/10W to simulate the field coil, or you could use a single high wattage resistor.

[PRR]

I mis-remembered. There is a 10+110+130= 250 ohm high-Watt resistor, near a dead spider, known to be good for dropping quite a few Volts at full radio current (more than your amp will pull). It may not drop a lot, but it is worth saving, if you didn't grind it out.

[blown240]

I am pretty sure I still have it.  I know the mounts for it are still there.  I would more it though, since my preamp board sits there now.  Thanks for the info!

[blown240]

Well I dont have that big resistor any more.  I thought I saved it, but I guess not.  While I was at it, I made a list of everything I need, and what I have.  I have alot less than I thought I would.  Oh well, time to make an order unless I can substitute...

Here is my schematic:



Here is the list of what I need:


Resistors   Quantity   Capacitors   Quantity
820                2               0.01         1
1500                2              0.1         2
2700                1               0.25         2
4700                1              .0001         1
6800                1             .00025         1
100K                3              .0005         1
100K 5%        1               .002         1
10K                1             .1-200         1
1K 5W 5%        1            20-600P         3
1M                5              25-25         2
220K                3            25-50 +-         1
27K 1W 5%        1           40-450 +-         1
470 1W        2      
4700 1W        1      
470K                1      
47K                1      
56K                1      
56K 1W        1      
68K                1      
82K 5%        1      

And Here is what I have on hand:


Resistors   Quantity   Capacitors   Quantity
2.1                 2              .0005        8
680                 2            .001        3
.33 5w         1              .0047        3
1.5K                 2       .01uf        4
1.5M                 2            .022        3
100K                 6         .047 1000V    1
100K 2W         2       .047 630v        1
120K                 4        .1uf        5
150K                 5              .56uf        2
1K 2W         4             0.33uf        3
1M                 2            100uf        2
220K                 3       32uf-500v        2
22K                 2               470pf        5
250 10W         2          47pf        2
270K                40     50uf 500v        2
3.3K                 9      
3.9M                10      
39K                10      
470K                 2      
47K                 8      
5.6K                 3       
5.6M                 3      
56K                10      

Sorry its hard to read,  The columns were straight before I hit post.

[PRR]

That bias supply will NOT work. It makes NEGative 400V, you want -50V.

There's two typical supplies. One comes off a 50VAC PT lead maybe through a small resistor. The other comes off a 350VAC PT lead through a BIG resistor.

Since you don't have a 50VAC PT lead, you better find values for the other kind.

The 470K before the .0005 is leftover from a mix network. Since you are not mixing, you probably do not want this resistor.

[blown240]

I thought the -54 on the schematic was correct for the bias?    How do I figure out what I need?

Thanks for the tip on the 470k reistor

[kagliostro]

About bias from HV tap see page 5 of Sluckey Amp Scrap Book

(and also the other pages that are interesting)

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

Kagliostro

[PRR]

> I thought the -54 on the schematic was correct for the bias?

Yes, that is what you want.

But the 1K from a 300VAC winding will give -400V bias, NOT what you want.

[blown240]

OH Ok.  I see what you are saying.  Thanks again!

[blown240]

Since I have so much room in this chassis, I decided to do separate boards for the preamp and power stage.  I got the preamp board done this morning.  I know it isn't the cleanest board ever, but its my first one and I am pretty happy with it.

[blown240]

Late last night I finished wiring this amp up.  I got up early to fire it up and check voltages.

The good new is that it passed the smoke test, but my 6V6 tubes started glowing red hot.  But I think my voltages are all screwed up.

Here is what I got:

Filter cap 1:  314v

Filter cap 2:  307v

Filter cap 3:  307v

Filter cap 4:  169v

The B+ at the rectifier tube is 315v

The bias is -314.  This is from the tube side of the bias to filter cap 1.  I was not sure exactly where to check it at.  

I need to get the bias to around -50.  Should I wire in a pot between the PT and the bias?

Thanks!

[sluckey]

The bias is -314.  This is from the tube side of the bias to filter cap 1.  I was not sure exactly where to check it at.

Which side of filter cap 1??? Check the bias voltage between pin 5 of the 6V6s and chassis ground. If it really is -314v you'll need to replace the bias cap because it has been severely stressed. 

I need to get the bias to around -50.  Should I wire in a pot between the PT and the bias?

Did you forget to heed PRR's post? Your schematic shows a 1K resistor between the PT and the bias diode. That resistor needs to be MUCH BIGGER. Try 100K to begin with. Then change the value up/down as needed to get the desired bias voltage.

[blown240]

ok cool.  I have a 100k pot as well.  But I will start with the resistor to see where I end up at.

[blown240]

For some reason I am burning out the 1k 1watt resistor in between the PT and the Diode?  I never even got to try a different value.  They burn out as soon as I turn the power on.  Any ideas?

The Bias cap is shot too, it literally popped.

And should I be doing this with the tubes in or removed?  So far the tubes have all been in.

[sluckey]

I have a 100k pot as well.

Pull the 6V6s and don't put them back in until the bias voltage is right!

Forget the pot. Replace that 1K resistor with a 100K resistor. Replace the diode with a KNOWN good diode. Replace that bias cap. Now what is the voltage reading between the 6V6s pin 5 and chassis?

[blown240]

ok, I replaced the 1k resister with 100k, put in a brand new diode, and replaces the blown 25/50 cap with a 50/50 (it was all i could get).   Now I am getting -11 volts at pin 5 of the 6V6.

Also,  with all the tubes out except the rectifier.  I am getting 568v and filter cap 1,2 and 3.  And 528 volts at Filter cap 4.  Should there be that big of difference with the tubes in or out?

[sluckey]

Now I am getting -11 volts at pin 5 of the 6V6.

That's better. But -11v is not enough. You want about -35 or -40 on pin 5 of the 6V6s. Replace the 100K with a 47K and see what you have. Then change the value up/down as needed to get the desired bias voltage.

Also,  with all the tubes out except the rectifier.  I am getting 568v and filter cap 1,2 and 3.  And 528 volts at Filter cap 4.  Should there be that big of difference with the tubes in or out?

There can be a BIGGG difference with tubes out, but 568V is probably exceeding the voltage rating on your filter caps. If so you risk blowing them just like you blew the bias cap.

Reread reply #54 for my solution to bring the voltage down to managable levels.

[blown240]

I am having a really hard time getting the bias correct.

I have tried several different ways to get -35 to -40 volts on pin 5 of the 6V6

Here is what I have so far:

1.1k resistor by itself burns up instantly
100k resistor gives me -11 volts

I hooked 8 resistors in parellel and got up to -22.  Here are the resistors I used:  100k, 100k, 100k, 100k, 27k, 27k, 27k, 4.7k  
This should have gotten me a total of 2.74k

Then I tried 4 resistors in series: 250ohm, 250ohm, 470ohm, 470ohm = 1.4k.  This got me to -28 but the 250ohm resistors were literally smoking.

All the resistors I used are at least 1 watt.  Part of me thinks the problem is that my voltage without the tubes is so high, but if I put in a large resistor like was suggested, wont that drop my voltage with the tubes in as well.  Voltage with the tubes in is almost too low.  Will the bias voltage change when I put the 6V6 tubes back in and my B+ drops?

What am I doing wrong here?

[DummyLoad]

need pictures.

--DL

[blown240]

Here is the built before I tested it:



Here is the bias circuit currently.  The resistor isnt in it because I need to know what value it needs to be.  These is a resistor under the cap:



The outside:

[sluckey]

Will the bias voltage change when I put the 6V6 tubes back in and my B+ drops?

No. Not if it's wired IAW your schematic.

What am I doing wrong here?

I don't know yet, but I suspect something is not wired correctly. Look at the attached simplified schematic and make the two voltage measurements indicated. One meter lead needs to be on chassis ground. Be sure to use a 100K resistor. The Princeton Reverb uses this exact circuit and feeds 340VAC to the 100K and puts out -34V on the bias cap. The circuit is a proven one. I didn't just make up that 100K value. The 100K should not get hot.

While you're checking the AC voltage, also check the AC voltage on the 5Y3... pin 4 to chassis and then pin 6 to chassis. Report back with the readings. There are only 4 components involved. There's gotta be something simple amiss. If you can post some high rez pics we may be able to spot something.

PS... For anyone that has trouble looking at the bright green schematic, look at this one instead. The green schematic is a 6G6-B Bassman without the normal channel and the rectifier is replaced with a 5Y3.

[sluckey]

Your diode is currently installed backwards. The banded end connects to the "100K" resistor and the black end connects to the negative end of the bias cap.

[blown240]

[blown240]

Your diode is currently installed backwards. The banded end connects to the "100K" resistor and the black end connects to the negative end of the bias cap.

WOW, now we are getting somewhere!!!  I will reverse it and report back with the new voltages.

[blown240]

OK Diode is correct now.

Here are the voltages:

Pin 4 & 6 on 5Y3 = 412vac

100k to diode = 222vac

Diode to cap = -43vdc

Pin 5 on 6V6 = -35  !!!!!!!!   SWEET!!!

[DummyLoad]

BTW, what's a "blown 240"? volvo with a roots blower?

[sluckey]

Diode to cap = -43vdc

Pin 5 on 6V6 = -35  !!!!!!!!   SWEET!!!

Those two voltage readings should be the same!

[blown240]

BTW, what's a "blown 240"? volvo with a roots blower?

EXACTLY!!  It was the first car I built.  13psi with 13:1 compression.  Had to run it on 116 octane or it would ping. Watch this vid:

  http://youtu.be/C3UzMk2ViY0

Those two voltage readings should be the same!

Is that a problem?  There is a resistor in parallel with the cap too,  wouldn't that change the voltage some?

[PRR]

> Is that a problem?

Yes.

What are you using for a volt-meter, something you bought for the Volvo? 1K/V needle-meter? Or a proper digital meter?

[blown240]

I am using a Fluke T5-600 voltage, continuity, and current tester.

I am using a 50-50 cap instead of the 25-50 that is in the schematic.  Could that have anything to do with it?

[sluckey]

I am using a 50-50 cap instead of the 25-50 that is in the schematic.  Could that have anything to do with it?

No. But mentioning that cap reminded me of something I forgot to mention... All the experimenting with all those resistors has severely stressed thatr bias cap. E-caps don't like reverse polarity voltages. Even if the cap seems OK now, I'd still replace it just because I know it's been stressed. This bias circuit is such a simple circuit that it's easy to think that it's not very important. But if that cap fails you risk losing your output tubes and maybe even some transformers. Replacing that cap is real cheap insurance.

[blown240]

ok, will do.  I will get one tomorrow.  But what about the difference in voltage?

[sluckey]

But what about the difference in voltage?

The only thing between the diode and the tube grid is a 220K resistor. No current flows thru the 220K, so no voltage can be dropped across it, therefore the voltage would be the same.

Your tester is the cause of the difference. Your tester has a 1meg input resistance. When you connect the tester to pin 5 now you create a voltage divider and current can now flow thru the 220K causing a voltage to be dropped. You basically have a 220K in series with a 1meg (your tester) across a 43 volt source. Doing a little math yields 35v dropped across your meter.

So, don't be concerned about the difference. It's legit with your tester. You may want to get a meter that is more appropriate for this type work.

Your next big challenge should be to get the B+ lowered to a safe value. You are risking blowing those big blue filter caps.

[blown240]

But what about the difference in voltage?

So, don't be concerned about the difference. It's legit with your tester. You may want to get a meter that is more appropriate for this type work.

Your next big challenge should be to get the B+ lowered to a safe value. You are risking blowing those big blue filter caps.

Very cool, thanks for the info.   

If I drop the B+ down with a large resistor, will it effect the voltage with the tubes in?  I think the voltage with the tubes in is already a bit low.  If it wont effect it, what value should I begin with to try to drop it.  And does it just go between the 5Y3 and the first filter cap?

[sluckey]

Before doing anything to drop the B+, re-evaluate now that you have a reasonable bias voltage. Put all the tubes in and recheck voltages. I'm mainly interested in the DC voltage readings on the 5Y3 pin 8, 6v6s pin 3, and 6V6s pin 4. Measure quickly and be prepared to turn the amp off if tubes glow red or any other odd thing turns up.

What are the new numbers?

[blown240]

5Y3 pin 8 = 485vdc

6V6 pin 3 = 474vdc

6V6 pin 4 = 483vdc

Bias is -35

The tubes are not glowing, except for the filaments, and nothing smoked!

[sluckey]

Those voltages are way too high for 6V6s. Find that big power resistor from the original radio and I'll give you some ideas using it to drag the B+ down. If we can get the voltage down to 400-425 range (Deluxe Reverb territory) then some good 6V6s will handle that OK. Find the resistor in this pic...

[blown240]

ok I found that resistor and measured the resistance.  It still works. Now what?

[DummyLoad]

ok I found that resistor and measured the resistance.  It still works. Now what?

put in series with the PT high-volt secondary center tap. IOW, lift the center tap lead of the high volt secondary off ground and connect it to one end of that resistor; the other end of the resistor connects to ground.

make sure that that resistor is bolted in well to the chassis - it's gonna get really hot - so hot you'll probably get a good scent of that new paint baking...

peace,

--DL

[blown240]

Sweet.  Hopefully I can get that done tonight after work.  I will post the new voltages...

Thanks!

[sluckey]

Give DL's suggestion a try first. Then try this...

Temporarily connect that 7750Ω between the 5Y3 pin 8 and ground and measure the voltage from 5Y3 pin 8 to ground. (Turn the amp off as soon as you get the voltage reading.) What is the B+ reading with the 7750Ω in place?

 

[DummyLoad]

sim shows about 40-45V drop, so neighborhood of 440V?

[blown240]

I will get answers to both of you guys tomorrow.  I just got home from an 18 hour work day and a 450 mile drive. 

Thanks for the replies though!

[PRR]

> sim shows about 40-45V drop

Simming what? Do you have good data for winding resistance of a Zenith PT?

[DummyLoad]

> sim shows about 40-45V drop

Simming what? Do you have good data for winding resistance of a Zenith PT?

e-s-t-i-m-a-t-e. SWAG. i'm new at it, but i i have figured out it's not an exact method.

--DL

[blown240]

OK, here we go.

Starting voltage this morning was 485 at pin 8 of 5Y3

Sluckey's idea got me to 450v

Dummyload's idea got me to 394v

So what is better?  Voltage a little high, or voltage a little low?

And how hot is this resistor going to get?  The best place to mount it will put it about 1/8th inch off of the wooden box of the radio and about 1/4 inch from a filter cap can.