SSS Cathode Follower and Bias Voltage in a 50W Fender Bassman Converted to Vibroverb

[Krinkle]

I know the post PI Cathode Follower and Bias have been discussed a lot on this forum, and I'm pretty sure I've searched and read everything here, so here goes. I think there are a lot of people here with both knowledge and experience with this circuit, please help.

I have a 50W Bassman head converted into a Vibroverb circuit, single channel, trying to get some SRV tone going. This leaves me with one unused preamp socket/tube. I wondered what I might do with that until I was searching for info on SRV amps and noticed the info on the SSS here. I decided to add the CF to my build. I have the CF circuit added, and a bias circuit whipped up, but I'm having trouble with a few things;

- I see mhartman's layout and I can't see how the bias circuit would work. If you calculate the bias voltage you should be getting about 1.41 times the AC coming off the one side of the PT. At a value of approximately 300VAC, that would be 423VDC. I just don't see that working. If anybody has connected that straight up and it worked fine please let me know, I'm scared that I'll fry something.

- Regarding above, I've read that the voltage at the cathodes should be around +75VDC as this provides a lower relative voltage across the tube with a high voltage on the plate, thus saving the tube from frying due to the 300VDC rating. How can +75VDC work when you need a negative voltage on the screen to bias the power tubes, which is essentially the same point in the circuit? I've searched for threads with posted voltages but I've seen conflicting information. Plus it's hard to sort out if some of these layouts and schematics are ones that have worked because they all seem to have disclaimers on them saying there could be errors. Quite the merry go round when I'm searching.

- I bought a Hammond 120:120 PT (229A120) at a surplus store to make a bias circuit as suggested in another thread here (and it's not 1:1, it gives me about 180VAC out, so I get 253VDC, but that's another story) and when I connected the circuits, the power tube bias current keeps increasing, I turned off the amp at 70mA, and my calculations indicate that it should be biased at about 46mA per tube.

- I bypassed the CF and the bias still kept running away on me. I read some other threads and this led me to removing the 470k resistors to ground off the screen resistors. Works, but I'm right back to where I started. I'm really suspicious of the 470k's but it seems that some people here have them installed and working, very confusing.

Help!!!!!

[larsvictor]

Krinkle,

here are two snapshots from the MHartman schematic. There is a dropping resistor between the HT tap and the diode (3k3). The DC voltage at the negative terminal of the bias filter cap is approx. -320 V. Of course this circuits works only with a center taped PT secondary. The hand drawn schematic is a cutout of the original schematic of SSS #002 with the voltages at different nodes. Notice that the voltages are a bit lower than they are supposed to be due to the lower wall voltage in Japan.

Of course the voltages of cathodes and grids must be negative. In my amp its -50V VDC (grids) and -51 VDC (cathodes).

You can go with an extra transformer. If you have -253 VDC bias voltage you have to replace some of the resistors in the bias network (the 820k) and the cathode resistors with lower values.

Lars

[Krinkle]

Thanks so much for the reply Lars. It's funny because I looked at the Japanese schematic today and it has a large bias voltage written in there that I didn't notice before. When I take into account that the input AC was 100VAC, the voltages values make sense if you extrapolate to 120VAC.

I made a new bias board and I am going to tap off of the PT HT (it does have a CT) instead of using the Hammond.

I think the biggest problem I had was the 470k resistors. Power tube currents were running away when I had those in the circuit. They aren't shown on the Japanese schematic so I'm just going to leave them out. I'll know in an hour or so if it works, when I get home and connect it all up.

[Krinkle]

No go. Current running away again. I'm baffled here. Lars, are you using the 470k resistors? Also, what is the voltage on the plates of your CF 12AX7?

[Krinkle]

Just figured out my latest bonehead move.

I replaced the Fender bias board in the amp with a copy of the SSS02 bias circuit, 3k3 instead of 470R, higher voltage rated cap, with the intention of supplying it with one tap of the HT PT........except I used the original bias tap wire. I measured the bias voltage, -60VDC. I make no wonder the current was running away like crazy. Holy cow. Normally I would measure, double check, etc, but my wife called for a pick up from the train just as I was finishing soldering and I rushed it, lesson learned (again and again).

Everything looks good now. I'm biased a little cold but that's easy to sort out. Even though it's a little cold it still sounds great, VERY clean.

I don't have the 470k resistors installed, do I need them??

Thanks for the help Lars.

[larsvictor]

Hello Krinkle,

glad you solved it.

I have +457 VDC on the CF driver plates (plate to cathode voltage is > 500VDC !!! )

I had to implement an additional resistor to get the bias in the desired range (pic 1). My PT had no CT. I´m using a 230:230 VAC transformer with FWB Rectifier for the raw bias supply (pic 2).
I have the 470k resistors installed, but I know some builders didn´t. I assume they are not necessary.

Lars

[sergio]

Take in mind that CF schematic #002 is unconfirmed!! 500V or so for plate-cathode in a 12ax7 is not good for a correct operation!

[larsvictor]

Thanks Sergio,

I know that. Datasheet allows max. 300 VDC. How did you do it ? I know you´ve built this amp too.

Lars

[martin manning]

Lars you could convert your 230vac to a bipolar supply at +/- 160vdc and power the cf stage with that alone. Krinkle you could use your existing bias tap with a quadrupler like I posted a few weeks ago. http://ampgarage.com/forum/viewtopic.php?f=4&t=29882 A link to the separate transformer version is in that thread too.

[larsvictor]

Thanks Martin,

I've seen your circuit and it's a great solution for that stage. My 230VAC PT has a center tap - perfect for your circuit (115 VAC rectified = 160VDC). I just wanted to give the "original" (?) circuit a try. If it kills the tube, I'll change the cf stage. My amp is not point to point wired, I made PCBs. So it's a little more work to change things afterwards.

Lars

[Krinkle]

Thanks for all of the input everybody, very appreciated.

I have converted the rest of the circuit so it has gone from a 1968 silver faced 50W Bassman (it was screwed up, somebody modded it before me) to a Vibroverb/Bassman to a full 002 build attempt now, using the "May Contain Errors" schematic (using modded power supply resistor values and minus the reverb as I didn't have enough tubes, but I have a Catalinbread Topanga and I'll fight anybody to the death who says they can tell the difference between it and real spring reverb ). This amp tone is far more to my liking than the previous incarnations.

I used Dumble's bias circuit (off one side of the HT with 3k3 resistor and cap) and added a 25k pot in series with a 100k resistor in place of the 130k and it was easy to bias. I don't really want to tear it apart anymore as I'm enjoying the tone so much so I'll keep it and see what happens. I don't have the 470k's installed. If they are crucial somebody please let me know. How did they get in the schematic anyways, I don't see them in the original hand drawn version?

My CF plate voltage is in the 300's, well under 400VDC, I can't remember the exact value right now and I don't have my notes with me. The power supply schematic that I used had an extra 27k resistor off of the PT screen node to provide a lower CF voltage.

I notice that the CF 12AX7 flashes a little when I power on the amp. I read somewhere about the voltage possibly stressing the tube at power on, can't remember where. If the tube blows I'll look to see if there's a soft start circuit that's not too complex that can keep that from stressing the tube.

[Krinkle]

One thing I forgot to mention is that I am not using the inductor in the filter circuit and the 100k was omitted from the MCE schematic so I didn't use it either. Anybody know why the 100k didn't make it to the MCE schematic?

I can hear the difference when I turn the Bass filter and I like it, but I can't really hear a difference in the High filter and I wish I could. I had read that the inductor makes no difference, but what about the 100k? Anybody try with and without? Anybody seeing big changes when they switch the High Filter?

[markusw]

One thing I forgot to mention is that I am not using the inductor in the filter circuit and the 100k was omitted from the MCE schematic so I didn't use it either. Anybody know why the 100k didn't make it to the MCE schematic?

I can hear the difference when I turn the Bass filter and I like it, but I can't really hear a difference in the High filter and I wish I could. I had read that the inductor makes no difference, but what about the 100k? Anybody try with and without? Anybody seeing big changes when they switch the High Filter?

If you build the filter section like in this schematic (skip the inductor)

the high filter will work.
I built the filter section exactly like shown in the above schematics as a stompbox and added it to the loop of a D-style amp. Works nicely.
Didn't check if there are any differences to the "mce schematic" though.

The high filter basically is a 6-position bright switch (think of the 820k/100k combo as a volume pot set to about 50%).
The low filter basically is a 7-position deep switch mixed to the "820k/100k volume pot" via the 270k (like the usual deep switch).
The 100k that "was omitted from the MCE schematic" together with the 10n bypassing the low rotary switch defines the corner frequency of the RC low pass filter. The output impedance of the previous triode stage has to be added to the 100k when calculating the corner frequency. W/o the 100k the corner frequency will be lower.

[Krinkle]

That's an excellent explanation of that circuit, thanks markusw! I'll add the 100k and see if it makes a difference in the sound.

Another comment I'll make about my build is that I don't hear any changes in tone when I activate the bright or the deep switches.

I double and triple checked my layout and my actual build but I'm going to check again.

[markusw]

The deep and bright switches are more noticeable at lower volume settings.
However, also with the volume pot at 5 or 7 they clearly change the sound.