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Old 5th July 2009, 08:32 AM   #1
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Default Correct OPT and other adjustments on old Bogen amp

Hello all. I have an old Bogen AP30 which blew an output transformer primary. I got this amp from my uncle who had it since new. It's been around the world literally but was working fine when I received it about a year or so ago. I replaced all the electrolytic and tubes since it had been sitting for so long and the output tubes tested low.

It was originally configured for 7408's which I replaced with 6v6's. I also replaced a lot of the old sprague film caps with new panasonic polyprop caps as well as all the old carbon comp resistors with metal film. The amp was looking and sounding pretty good for an amp with such a weird design.

Some major modifications were the elimination of the tone circuit (and corresponding make up gain stage) and directly coupled the switch output to the grid inputs of V4a&b. I also connected the suppresser grid to the plate to run the output tubes in triode mode instead of tetrode.

I'd like to do a couple of things to the amp that I'm not sure how to do. First of all is obviously the replacement of the output transformers. I'm looking at the CXPP Edcor's mostly because of price, but I've heard nothing but good things about these OPT's. I'm not sure what primary impedence I'd need for this particular setup, but I'd also like to run 6l6's in there primarily, these should net higher gain, correct?

Finally the bias circuit is the weirdest I've ever seen. It uses the heaters of the 2 phono preamp tubes for the cathode resistance with a 250uf bypass cap. I'd like to be able to adjust the bias for the tubes. How could this be implemented?

Forgot to add that I have an external power supply that I use for all the filaments to provide DC to all. I've included the schematic to clear things up. Any help is appreciated. Thanks.
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Old 6th July 2009, 04:31 AM   #2
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Using V1 and V2 heaters for the output tube cathode resistors is a fairly common trick in that vintage of amplifier.

It is a way of getting a "free" DC heater supply for these 2 tubes.

V1 and V2 have 12.6V @ 150mA rated heaters.
That means that the output tube design bias point is 2 x 12.6 = 25.2 Volts for a total idle current (4 tubes) of 150mA. That is 37.5mA each.

All that is a bit "iffy" particularly when you start changing output tubes to something else. 7408 is equivalent to a 6V6 so that should not be too much of a problem.

6V6 data suggests -19V bias for 35mA idle so I would think that with 2 x 12.6V heaters of V1 and V2 in series you are not going to get 35mA per tube and therefore not going to get full heating to V1 and V2. Total heater current in V1 and V2 will be a bit less than 150mA and so voltage drop across those heaters will be a bit lower and output tube idle current then increases a bit - so these things tend to balance out a bit.

If the output tubes are under biased you can add adjustment with a pot in series with V1 and V2 heaters BUT in your case the tubes are likely to be over biased rather than under. I would'nt worry about it, just leave things as is. You do need to use a matched quad of output tubes for best performance.

If you must make mods - then since you say you have an external DC supply for all heaters you could:
1) Wire V1 and V2 heaters to teh external DC supply.
2) add a 560 Ohm 2W resistor between each output tube cathode and 0V and bypass each 560 Ohm resistor with it own 100uF / 63V capacitor.
That individually cathode biases (auto bias) each output tube and it is then far less important to use matched tubes.

Hope this is useful.


For new output trannies - anything with a 8K Raa and 15 Watt rating should be fine.
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Old 8th July 2009, 01:18 AM   #3
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You hit everything right on the nose. I know the answer to my question will be "use ohms law" but could you walk me through how you came to the values for the cathode biasing? I realize you figured out the current draw and all from the 12ax7 datasheet, but I'm basically wondering how to calculate the resistor/cap values for this.

Thanks for humoring a newbie. You've been much help.

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Old 8th July 2009, 04:22 AM   #4
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First we need to look at some application data for the 7408. We quickly draw a blank - there is application data about for single ended use but I could'nt find anything on push pull use. We also know that it is equivalent to a 6V6 so look at application data for it instead.

That is where I got the -19V bias for 35mA from.

Lets start with the 6V6 Anode Dissipation rating of 14 Watts.
I like to run 6V6 at about 75 to 80% of maximum rating so that the tubes last a bit longer.

That means run the 6V6's with a dissipation of say 11 Watts.
From the schematic the anode supply voltage (to the centre tap of the output tranny) is +350 Volts.
There will be about +20V on the cathode developed across the cathode bias resistor. With the grid at DC 0V that means the Voltage from grid1 to cathode is -20V, that is the bias voltage.

Anode to Cathode Voltage is 350 - 20 = 330 Volts
For an anode dissipation of 11 Watts then the idle current we want is 11 / 330 = 33.3 mA. This is the cathode current which is actually anode current + screen current so round it up to 35mA quite safely.

From that application data again we see that with Voltage of 285V and a bias of -19V then we get 70mA idle (that is the value for both tubes) or 35mA each tube.

We are actually running about 330V not 285V so increase the bias voltage proportionally.
We then want 19 x 330 / 285 = 22 Volts of bias

So we need to develop 22V across the Cathode bias resistor when we have 35mA flowing
22V / 35mA = 629 Ohms.

620 Ohm resistors can be bought but 560 Ohms or 680 Ohms are More common values. Since we were VERY conservative in our tube power calc. I suggest you use 560 Ohms.

The resistor will dissipate Volts x Current = 22 x .035 = approx 800mW of power, so use 2 Watt metal film or even a 5W Wire Wound resistor for these.

The bypass capacitor
The original bypass cap was 220uF. That would be across about 1/4 of the resistance we have when using separate cathode bias resistors on each 6V6 - so about 1/4 of 220uF across each resistor would be fine. That suggests about 55uF. Common Values are 47uF and 100uF. 100uF are cheap and will give slightly better bass response that 47uF.

I hope that this is what you wanted.

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Old 8th July 2009, 08:56 PM   #5
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Ian, again you hit the nail square on the head. I totally see what you are doing and it makes complete sense to me.

I've related what you have taught me to what I see on the datasheets and feel more confident now when reading them. I really appreciate you breaking it down into bite size pieces for me.

I'll conclude with one final question. I didn't see anywhere on the datasheet that dictated the bypass cap value. I didn't know if there was a rule of thumb for this, or a ratio/proportion with the cathode resistor. I know in this case the value was given so it could be cut in 1/2 or 1/4 depending on how it's connected. I'm wondering say if i wanted to change the output tube to something else how would I calculate this.

Ian thanks again, your explanations are wonderful and very easy to understand.

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Old 9th July 2009, 12:16 AM   #6
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Generally you will not find bypass cap details in Application Notes or Data Sheets.

The size of the bypass cap determines the bass response to a fair degree so you want to make it reasonably large BUT there is a trade off with recovery from overload.
At any instant the actual grid 1 to cathode voltage is the bias voltage + the signal swing. So if we have -22V of bias and (for example) a 44 Volts peak to peak signal applied to the grid then the actual g1 to cathode voltage swings between 0 and -44V. As this voltage approaches 0V or even goes positive on the positive peaks of the input signal then the tube draws grid current, that is the grid 1 to cathode looks like a forward biased diode and the bypass capacitor charges up increasing the effective bias voltage and reducing the idle current until that extra charge is bled away. In the worst case this can cause a tube to cut off. This is known as blocking distortion. The larger the bypass cap the longer it takes to bleed away this extra charge. That is, the recovery from overdrive takes longer. So we have a compromise. Too small and you don't have enough bass response, too big and you can have longer recovery from overload. That is why most guitar amps use fixed bias rather than cathode (auto) bias.

In this instance I simply decided to keep the effective bypass capacitors much the same as the existing circuit. So that bass response and overload recovery times are much the same as before.

We said that that the V1 and V2 heaters in series is effectively a 25.2 Volt / 150mA = 168 Ohm resistor.

The bypass capacitor is in parallel with the cathode bias resistor.
What is less obvious is that there is an effective "looking in" to the cathode impedance to 0V which is also in parallel with the cathode bias resistor. Its value (for a pentode) is approximately 1/gm which for a 6V6 is approximately 180 Ohms.

So in the existing circuit we have 168 Ohms in Parallel with 4 lots of 180 Ohms. Calculating the effective resistance gives a value of 36 Ohms. So that original 220uF is in parallel with 36 Ohms.

36 Ohms and 220uF gives a Time Constant (R x C) of 7.9 milli seconds.

With separate cathode bias resistors for each tube we have 560 Ohms bias resistor in parallel with that 180 Ohms "looking into the cathode" impedance or 136 Ohms.

For that same 7.9 ms time constant the capacitor needs to be
0.0079 / 136 = 58uF

You can see that with all the fancy analysis we get much the same answer as when we simply said that the bias resistors are 4 times higher so the caps can be 4 times lower. The argument would have been more accurate if we had said we have 1 cap for each tube instead of 1 cap for 4 tubes so the cap can be 4 times smaller.

So as I said before either 47uF or 100uF will do. I suggested the 100uF since amps of that vintage tended to use values on the low side, as higher value caps were quite expensive. Today 47Uf and 100uF are likely to be around the same price.

Typed this up in a hurry before starting work so I hope it makes sense.

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