Build Suggestion

[ylelevier77]

Hi there. Kinda new to amp building and I was looking for a build suggestion. I attempted the Blues Jr conversion build and it didn’t come out to my liking. So I’m looking for a new build suggestion. I’d like to try something along the lines of a Plexi style amp in the Blues Jr chasis. My PT is an Allen Amps TP26 which is a dual voltage secondary PT that can deliver a B+ of either 380V or 440V. The manufacturer has been very clear that this PT needs to go into a full wave bridge solid state rectifier. My OT is a TO20B also by Allen Amps and is suitable for driving either EL84s or 6V6s at 8ohms. The chasis has five tube sockets designed for 3 12AX7s and two EL84s. Looking forward to hearing back on new build possibilities from some experienced builders here :-)

[silverfox]

Can you think of a guitarist, the sound of the amp, you like? What are your personal aspirations tone and style wise? What sort of features are you interested in; Re-verb, effects loop, high gain??

What is it about the first amp you built that did not appeal to you?

silverfox.

[ylelevier77]

I’d definitely like something like a tight smooth classic overdriven tone. Just a one trick pony classic rock amp. AC/DC tones would be ideal. Really no need for tremolo or reverb. I was thinking of using the reverb pot location for a presence control. I have a couple of Fenders so I was thinking of something with a tighter feel. Especially since I’m locked into using a solid state rectifier. That’s why I was thinking of something like an early 70s plexi style circuit. What I didn’t like about my previous build was that I was getting way too much gain. Like fizzy style gain with the preamp vol level at 3. Could have been that the non stock transformer was feeding too much voltage into the circuit. Not sure but in any case I figured I’d try the forum for some inspired and educated direction. Any ideas?

[tubenit]

How about a HoSo56 using 6V6's?    5879 pentode for V1 and a passive effects loop & PPIMV.

With respect, Tubenit

[tubenit]

Or a Tweed OD lite with 5879 overdrive.  You could either use Hoffman's relay switching from clean to OD ........... OR  a simple mini-toggle

There is a ExpressSCH editable layout that can be easily modified for this schematic.

Both the rhythm and lead work on this tune was recorded with a Tweed OD Lite.

https://www.soundclick.com/html5/v3/player.cfm?type=single&songid=9575220&q=hi&newref=1

With respect, Tubenit

[darkbluemurder]

I second the HoSo suggestion. I have built one starting out with EL84/6BQ5 tubes but later converted it to 6V6 tubes. If you want less gain than the HoSo has to offer and since you are into AC/DC tones, 6V6 Plexi would be good. With a pair of 6V6s it is loud but not overbearing.


Cheers Stephan

[ylelevier77]

Yeah that Hoso looks like a nice simple design and I love the ppimv. Is there a particular kind of amp that it's modeled after? Can I run it with EL84s and is there a wiring layout available? I’m still a bit of a noob to building so schematics still have me scratching my head at times.
 With regards to the 6V6 Plexi, is that the one on Hoffman’s site and could I substitute the full wave bridge rectifier in place of the tube one? Could I still use EL84s? Thanks for the help.

[ylelevier77]

Ok, I just peeped the 6v6 plexi and I love it. Sound clips are the tone I’m looking for and it uses a solid state rectifier. My only concern is that Allen Amps suggests a full wave bridge rectifier that uses four diodes and it looks like the plexi only uses two. Wondering if there’s a way to work around that? I’d really like to use the PT I have and not drop any more cash on this build other than maybe a turret board and a few components.

[terminalgs]

Another amp to look at is the 18W stout. http://el34world.com/projects/18WattStout_TMB.htm    you can order a board with turrets installed (or not if you prefer).  The page has build order pictures, layout suggestions,  etc..  and there are lots of threads on this forum about building the amp.   

As far as the power transformer and rectification goes, you might google 'valve wizard Rectifiers'.   That will show you why your PT requires a bridge rectifier and how you might adapt it to the other amps that use a center tapped secondary.

[shooter]

I just peeped the 6v6 plexi and I love it.

+1
I did a variant of this using a clubman PA section, of all the prototypes and full builds, this makes the top.

It's well documented, I would suggest using "whatever" chassis was used for the build data you're working from, like Dougs Stout chassis, I used a 17" but it's what I already had.

[ylelevier77]

Yeah I’m hoping I can use my PT and OT for the 6v6 Plexi. I think I’d also need to substitute the two diodes coming off of the secondary for the bridge rectifier Hoffman uses in the Blues jr conversion. Again, I’m somewhat locked into using the Blues Jr. chasis, cabinet, and 12” 8ohm speaker. Also Allen Amps rated his dual voltage TP26 PT I’m using at 380V B+ or 440V B+. So I’m assuming all I’ll need for the plexi build is the bridge rectifier, a choke, a cap can, turret board, and any components I’m missing (mainly the caps and any resistors rated over 1watt). Right?

[PRR]

> need to substitute the two diodes coming off of the secondary for the bridge rectifier Hoffman uses in the Blues jr conversion.

Use the same diode scheme as the PT originally used.

PTs can be wound say 300VAC for a Bridge, or 300-0-300V for a 2-diode plan; either way ~~400V DC. Mix-n-match, you get half or twice what you expect. When a "400V" power supply comes up 800V (happened to me ), the 450V caps get very excited.

[ylelevier77]

@PRR So you’re saying go with the bridge rectifier that my PT originally used in the Blues Jr. conversion build instead of the two diode scheme shown in the plexi build, right?

[sluckey]

The manufacturer has been very clear that this PT needs to go into a full wave bridge solid state rectifier.

You must follow this advice.

[ylelevier77]

Okay so I decided to revisit the old blues jr conversion build and take some thorough B+ measurements before I scrap it and move on to a Plexi style build. When I measured VDC off of the node consisting of the rectifier and the B+ red wire from the OT primary, I get 338VDC. In Hoffman’s schematic for the Blues Jr. conversion 340VDC is the desired B+. So I’m right there. However when I connect that node to the rest of the circuit the VDC jumps to 420 VDC! Am I missing some me fundamental understanding? Should the voltage jump up that much?

[shooter]

Should the voltage jump up that much?

Um, NO, unloaded Volts should be higher than loaded volts, so, yep, there is a mis-understanding somewhere

[ylelevier77]

That’s what I was afraid of. Any ideas what could cause a jump like that?

[PRR]

> I measured VDC off of the node consisting of the rectifier and......

Was the Main Power Cap connected at this time?

If not, then it isn't "DC" but a madly bopping wave which does not read well on some meters.

WITH the main power cap it gets steady near the peak of the bop-wave. (However I would expect more rise- maybe the meter was confused.)

[ylelevier77]

No. The main power cap was not connected. I measured off off where the rectifier + output connects to the B+ wire from the OT’s primary. If I add the filter cap and/or a 2.2k 3watt resistor that’s part of that node, the voltage jumps up. I’ll try to add a pic and circle the node.

[sluckey]

No. The main power cap was not connected. I measured off off where the rectifier + output connects to the B+ wire from the OT’s primary. If I add the filter cap and/or a 2.2k 3watt resistor that’s part of that node, the voltage jumps up. I’ll try to add a pic and circle the node.

That's exactly what should happen. Nothing wrong here.

[ylelevier77]

Any suggestions on how to bring the voltage down then?

[sluckey]

Any suggestions on how to bring the voltage down then?

You said earlier, "My PT is an Allen Amps TP26 which is a dual voltage secondary PT that can deliver a B+ of either 380V or 440V."Sounds like you simply need to use the lower voltage secondary leads.

[ylelevier77]

They are the lower voltage leads. 

[sluckey]

Measure the AC voltage across the secondary leads at the bridge. What have you?

[ylelevier77]

Okay. So am I to measure at the positive terminal end of the rectifier or do you want me to measure each AC point at the rectifier?

[pompeiisneaks]

You need to read the pre rectification voltage there where it says AC yes.  The Full Wave Bridge Rectifier increases voltage a bit (1.4X as I recall) so sluckeys' wanting to see what the output at the transformer tap is. 

~Phil

[sluckey]

Put the black meter lead on one "AC" terminal on the bridge. Put the red meter lead on the other "AC" terminal on the bridge. Meter function set to measure AC volts. What numbers are in the display?

If your meter leads are a different color, read the instruction manual.  "wink"

[ylelevier77]

Okay. Gotcha. I got a reading of 302VAC.

With red and black probes ;-)

[sluckey]

302VAC will give 427VDC at first filter cap with no tubes plugged in. That's fine for 6V6s.

[ylelevier77]

Unfortunately I’m checking voltages for my old Hoffman Blues Jr. conversion build. It uses EL84s. I wanted to give it one more go over before I scrap it and go for a new build. Right now I’m leaning toward the Stout TMB. But like I said I’d hate to give up completely on the blues jr before moving on. Figured you all know best.

[ylelevier77]

I used a voltage drop calculator to come up with the proper resistor to bring down the PTs secondary to the desired 256VAC from Hoffman’s blues jr schematic and I came up with a 75ohm 3watt resistor for between each secondary lead and the bridge. On a scale of 1-10, how bad of an idea is that?

[sluckey]

I've done that before. Try it and see. The absolutely best option would be to put the original PT back in the circuit. That beefed up Allen PT ain't doing anything but causing you grief.

[ylelevier77]

Truer words....

[purpletele]

Hi there. Kinda new to amp building and I was looking for a build suggestion. I attempted the Blues Jr conversion build and it didn’t come out to my liking. So I’m looking for a new build suggestion. I’d like to try something along the lines of a Plexi style amp in the Blues Jr chasis. My PT is an Allen Amps TP26 which is a dual voltage secondary PT that can deliver a B+ of either 380V or 440V. The manufacturer has been very clear that this PT needs to go into a full wave bridge solid state rectifier. My OT is a TO20B also by Allen Amps and is suitable for driving either EL84s or 6V6s at 8ohms. The chasis has five tube sockets designed for 3 12AX7s and two EL84s. Looking forward to hearing back on new build possibilities from some experienced builders here :-)

ylevier77,

I bought a TP26 from David Allen and he never mentioned nor is it published that I can find that you have to use a Full Wave SS rectifier, unless I am completely missing something.

I bought it for a Princeton Upgrade which David recommended which uses a tube rectifier.

I read on the cut sheet to not use a 5u4 rectifier tube,but that is all I found as a restriction.

I am concerned now and I am going to re-check the spec's o the TP26.

Let me know where you are getting that rectifier info please.

BV

[sluckey]

I bought a TP26 from David Allen and he never mentioned nor is it published that I can find that you have to use a Full Wave SS rectifier, unless I am completely missing something.

You're missing one very important thing... The TP26 has no center tap on the HT winding. It's a replacement for the TP24 and it must be used with a FWB rectifier (or a voltage doubler). It cannot be used with a tube rectifier.

I read on the cut sheet to not use a 5u4 rectifier tube,but that is all I found as a restriction.

It also doesn't even have a 5VAC winding!

I bet you are thinking of the TP25.

[ylelevier77]

Of course. Here are pics I took of the original wiring layouts that came with my TP26. I zoomed in on the disclaimer regarding a full wave bridge (FWB) rectifier. I also coresponded with him via emails and he reaffirmed it needed an FWB. Let me know if you need anything else.

[sluckey]

Here's a quote from Mark Fowler on another forum. Mark is a straight shooter and knows this stuff...

Ordered TP26 from Allen Amps NEW! TP26 power transformer replaces the TP24. Same ratings and windings as before but with the addition of even higher voltage taps. Using a solid-state full-wave-bridge type rectifier, the TP26 will produce a B+ voltage around 370VDC at 150mADC which is great for EL84 based amps. Now with the new higher voltage leads (two red with white stripe leads) the TP26 can also produce a B+ voltage around 425VDC at 150mADC making is an excellent choice for larger octal power tubes such as 6V6s, 5881s or 6L6s. The TP26 is rated 120V to 280V/315V at 200mA and 6.3VCT at 4A with a 20V at 100mA brown to brown winding making it a great upgrade for the Blues Jr™ amp. 2" by 2-1/2" mounting centers, 4.25lbs, 3" tall above mounting surface. As used in the Allen Chihuahua, Classic 10 and Model LF amps.

[purpletele]

I bought a TP26 from David Allen and he never mentioned nor is it published that I can find that you have to use a Full Wave SS rectifier, unless I am completely missing something.

You're missing one very important thing... The TP26 has no center tap on the HT winding. It's a replacement for the TP24 and it must be used with a FWB rectifier (or a voltage doubler). It cannot be used with a tube rectifier.

I read on the cut sheet to not use a 5u4 rectifier tube,but that is all I found as a restriction.

It also doesn't even have a 5VAC winding!

I bet you are thinking of the TP25.

I think you are correct on the TP25.  I made a mistake on the Deluxe and now I'm spinning.  Time to go eat. 

Thanks for straightening me out.

[purpletele]

Of course. Here are pics I took of the original wiring layouts that came with my TP26. I zoomed in on the disclaimer regarding a full wave bridge (FWB) rectifier. I also coresponded with him via emails and he reaffirmed it needed an FWB. Let me know if you need anything else.

Thank you!

I think I panicked for no reason

[ylelevier77]

Well, those resistors did the trick on my Blues jr conversion build. Dropped the voltage to a very usable voltage. One things for sure, that TP26 is a beast. Definitely gonna go with the Stout build though. I figure I’ll be able to have a nice plexi tone AND room enough for a one tube reverb circuit.