Dopplesonde fixed bias

[sinusoids]

A. I built a pair of Dopplesondes with 5F6 preamps.  All is operational except the fixed bias.  As built, it had -20V B-.  With Cathode bias switched in, B- was -340V.  After trying lower R for the Bias supply I found with only 56k R, the bias was -94V with one set fixed and-56V with both sets fixed.  I now have one 56k R and 22uF C dedicated per tube set.  Both tube sets now have -56V max bias each.  They still interact quite a bit.  I can set bias for both V3+4 and V5+6 but if I have both in fixed bias the idle current for both increases quite a bit.  Any guidance here?  These current values include a switch to the grey 115VAC PT input tap – originally had used the black 125V tap.

B. Is my TUT bio editable?  I did not know that info would be included with every post – can I delete the bio for new posts?

[K O'Connor]

Hi

How are you generating the bias supply? It sounds like you have a CTed plate winding and are using high-value resistors in the usual way to make a bias supply. It is best with that type to clamp the first filter cap with a zener so the bias-set network does not require critical value.

We'll change the bio visibility.

[K O'Connor]

Hi

The bio is user editable, so you should be able to trim it down to whatever you wish. There does not seem to be an easy way to make the bio only visible to people logged in, or that they would have to click on your username to see what info you've posted.

[sinusoids]

Unless I have wired a mistake that I can't find, Dopplesonde bias and everything else is from from TUT5 CH 10 schematic and layout. The only difference is the driver, V1, has both triodes parallel and the filament center tap is biased to the same node as the concertina bias.

I'll edit it. I thought the bio was just for your ref when granting access. Mine has a lot of excess info.

[K O'Connor]

Hi

Disconnecting the raw bias supply from the bias pots for cathode mode takes away any divider action offered and B- is then unlimited. Add a zener as described - it should have been there to start

[sinusoids]

Should I add two? one each set of power tubes?

[K O'Connor]

Hi

if you are going to retain the redundan bias supplies then each needs a zener clamp. You should be able to use a single bias supply for both tube sets. Adding the zener makes the maximum negative voltage independent of the bias-set networks, leaving it open for you to modify the range of one network without it effecting the other... too much.

There is always a bias interaction with pairs of tubes, so the best tact is to set the firt by meter then set the second by ear for minimum hum. Doing a cross-zero between the push-pull pair may set DC idles the same, but this is not the same as the hum-balance point for the OT.

[sinusoids]

That FIXED it. 47V 1W ZR across both of the bias supplies. I have the second amp and will tack in 1 ZR where the 56k Rs meet and see what the difference is and report back.

Thanks!

[sinusoids]

I tried one B- supply for both power tube sets, with a 47V zener between the 56K Rs and with it at the ouput. Both of these would drop out when the second set of power tubes were switched to fixed bias. Not as dramatically as without the ZR. Rather than try different R values, I opted for two B- in parallel. This is stable on both amps. Thanks again. These took quite a while to build with the wires and switches. Nice to have full capability and lots of options. One tube set has 500R cathode bias, the other has 560R. I built them with 5F6 preamps and the 3 effective triodes can get quite the range of tones. One amp is to trade for a session 112 and a shadow 110 I finally had built. I will post a pic soon.

I cannot find an option to edit bio. Only the signature.

[K O'Connor]

Hi Sinusoids

Here are the steps for a Forum User to change their Bio.

1. Log in to theultimatetone.com

2. Click on "User CP" at top left

3. Click on "Edit Profile" from the menu that appears on the left - this will open the "Edit Profile" window

4. Your Bio may be edited under "Additional Information" at the bottom of the "Edit Profile" window

5. When done, click on "Update Profile" at the bottom of the screen

[makinrose]

I tried one B- supply for both power tube sets, with a 47V zener between the 56K Rs and with it at the ouput.  Both of these would drop out when the second set of power tubes were switched to fixed bias.  Not as dramatically as without the ZR.  Rather than try different R values, I opted for two B- in parallel.  This is stable on both amps.  Thanks again.  These took quite a while to build with the wires and switches.  Nice to have full capability and lots of options.  One tube set has 500R cathode bias, the other has 560R.  I built them with 5F6 preamps and the 3 effective triodes can get quite the range of tones.  One amp is to trade for a session 112 and a shadow 110 I finally had built.  I will post a pic soon.

I cannot find an option to edit bio.  Only the signature.

I've been following your post. I'm eager to see what it all looks like and hear more about the sounds you are getting!

[sinusoids]

Due to social distancing, the testing is pretty limited = my opinion for now. The triode/pentode and fixed/cathode switching has the most effect at cleaner settings. I can hear the bassman coming through but this has more overdrive.
I ended up with -72V for the B-, one for each pair of power tubes.

[sinusoids]

   
Grey chassis I built for myself 
   

   
I built an almost identical black amp for another guitar player that is very good at building cabinets.  I sketched this out from the DIY speaker cabs book and he built it.  He now is studying RSG, TUT and DIY speaker cabs.  This is the session 112 with legs and static casters in back.  Force 12 on the bottom.  Been playing this for months, can't believe how great it sounds.

[sinusoids]

   

This one has 4 and 8 Ohms out. 

   

Another cab he built is the Shadow 110 with legs and another TUT5 SOMA 84 I built in a harvest chassis. 
For my wife and her elec violin.  No treble control and the presence stays at max so that could have been eliminated as well.

[sinusoids]

Kevin, I’d like to power scale this. With fixed and cathode bias both being used at one time, would SV1, VCK and RBX installation work well?

Also, I imagine lots of us on this forum have recently spent more time at home in front of an amp. I have. It is nice to have this one to play with so many possible tones. Part of the reason for building it was to use it for trying out mods I have read about in the TUT series. The 5F6 pre is covered very thoroughly and the DOPPEL has many of the power amp mods built right in. Rewiring the 5F6 follower into parallel triodes was, by far, the most rewarding change. Kevin has offered this advice many times. This amp now has parallel triodes before and after the EQ. Also has shielded OPT wires to the speaker jack to minimize coupling/oscillation. Several Ra, Rk and Ck changes. I have not tried to change anything in the power amp, except to dial in the bias supply. I wish now that I had installed a few extra switches and pots on the front.

[K O'Connor]

Hi Guys

Yes, any time fixed-bias is present the Power Scale solution includes SV1 instead of SV2. Having switchable bias modes in the amp is very easily dealt with using a SPDT switch with grounded wiper, one end to the top of Rk and the other to SV1 J1-1, If there are individual Rks, add more poles to the switch to shunt them when in fixed-bias mode.

Have fun

[makinrose]

That's one fine looking rig!

[sinusoids]

Thank you very much.  Lots of drilling.  I need to fit another pot in. 

The cab is a work of art.  I was in the Lobby of the local tube store and asked my standard question when I run into people there:  "What are you working on"?  His responded that his project was a cab.   !!  We worked out a deal for a design from SPKR, with legs and casters.  I cannot believe how these sound.  Well, it might be expected, considering the source.