Metropoulos Forum

Hello All,

I have a '70 Traynor YBA-1 that I am converting to a JMP 50 and I am quite stumped at the moment. I have done tons of searches, I promise.

Background: I am using the Triode Electronics version of the 1987 layout and it has gone well up until when I have started to get voltages in step 1 of the testing instructions.

Here is what I have so far:
1)I am getting 114vac at the power switch.
2) Heaters are at 3.45vac each side (which is a little hot I know).
Turn the amp on to run (standby off).

I have implemented the Full Wave Bridge Rectifier per Hoffman's description. My PT has no center tap. That was my initial problem. However, I get 337 VAC across the two red HT lines. Rectified I only get 342 VDC. I know my diodes are fine. I test them and they are doing their job. But as soon as I flip the standby switch, the HT fuse blows.

I have also retained the separate bias tap from the Hammond PT and used it to feed the bias circuit. Those voltages aren't right quite yet but they are converted to DC Neg properly. This is no longer the case. I am using a leg of the HT like on most Plexi builds. I think I have a leaky coupling cap going to v5. Will test that later.

The biggest fish to fry as I see it is to get my HT rectification sorted out. I think I should be getting about 471.8 VDC after rectification.

Here are the pictures I have.





Pic removed for latest gut shots down below.

Pic removed for latest gut shots down below.

Pic removed for latest gut shots down below.

Pic removed for latest gut shots down below.

Paul

So I undid the bias supply tap going to bias circuit. I hooked up the the bias supply 47k resistor flowing into the diode from the right side of the full wave rectifier.

I was getting rectified 325vdc. But when I flipped off the standby, it would blow the HT fuse.

I undid ground from the full wave rectifier and my VDC dropped to 250 at the + side of the rectifier. When I flipped the stand by off, the HT fuse did not blow. I then went and tested the VDC at all the pins where they should be (3 on octal sockets, and 1 & 6 on the 9 holers) and my voltages finally got up to 250vdc and 205vdc respectively without any tubes in.

I think it's pretty obvious that I am hooking up the full wave rectifier incorrectly because I need that 325vdc at a minimum. With this Traynor, I would expect much higher rectified VDC, but it looks like 325vdc is all I am going to get.

Does anyone have any ideas?

What is interesting about the YBA-1 full wave bridge rectifier circuit is that it does ground on the negative side of the rectifier. But it is activated by the standby switch. The positive side goes to both positive sides of the 40uf can cap. The schematic shows I should get 440v at the positive side of the bridge rectifier.

I am thinking that I need to disconnect the remainder of the circuit and then see what I can do to get the B+ up to 440vdc. Then, once I get the 440vdc (or thereabouts), then I can send the current on down the line.

Any help out there?

And that is how Hammond themselves say rectification should be. Looks similar to the Traynor circuit.

So it seems to me that the negative goes to the negative side of the cap can and the positive goes to both of the positive poles of the cap can. Somehow I need to work in the standby switch and the HT fuse.

Sounds like you are only getting half wave recification (Vout=Vpeak/2)

I agree the bottom diagram of a brdge (with grounded neg. should give you Vout=Vpeak/0.707)

Presume you have checked your diodes, quality of grounding.

The photograph you show puzzles me as you seem to be sending the +ve red wire from your rectifier straight to the plates of the output transformer. I would normally expect it to go to your mains filter cans and standby switch then to the first choke wire/screens take off.

Is the picture misleading or has a wire been transposed?

neikeel wrote:Sounds like you are only getting half wave recification (Vout=Vpeak/2)

I agree the bottom diagram of a brdge (with grounded neg. should give you Vout=Vpeak/0.707)

Presume you have checked your diodes, quality of grounding.

The photograph you show puzzles me as you seem to be sending the +ve red wire from your rectifier straight to the plates of the output transformer. I would normally expect it to go to your mains filter cans and standby switch then to the first choke wire/screens take off.

Is the picture misleading or has a wire been transposed?

Thanks a million for responding!

I will take a better picture tonight (Pacific time) and post it up. I have redone the rectifier section. It is a bridge but it looks a bit cleaner.

The diodes are functioning properly according to my meter which has a diode checker function. One direction it shows passing .5 on each diode and the other lead direction it shows nothing. Which is correct as far as I know.

The picture is misleading you as far as the +ve. It is being sent to the HT fuse which is a 500ma fast blow fuse. With the ground connected to the -ve, I was popping the HT fuse relentlessly when I switched on the standby. When I lifted the ground, my +ve dropped to 250vdc from 340vdc before standby but grounded -ve. But at least when the -ve was ungrounded, I wasn't popping the HT fuse.

The way I have the +ve routed off the rectifier is: +ve to HT fuse, to the standby switch, then to positive lug of a 50/50uf cap can, to the other positive lug of a 50/50uf cap can, to the HT lug where the Output Transformer and one side of the choke hook up.

Now, the 50/50uf cap can is grounded to the chassis. But I think I need to have the -ve of the rectifier go to the 50/50uf cap can negative lug and remove the ground from the same lug. All of the diagrams of a full wave bridge rectifier show it has not going to ground immediately.

I think you are on the right track.

It is probably a good idea to switch the HT at the ac (ie take your ac from the PT to a DPST standby switch, pull your bias feed off this if your dedicated bias feed is not up to the job.

- may I suggest that you hook the mains filter cap up directly to the +ve of the recifier and the ground of the rectifier directly to to the cap ground - which goes to its own dedicated chassis ground lug.

- then put your HT fuse in line taking the first lead of your choke and HT out to the board B+ line

As you are if you are using a fast blow fuse you may be hitting it with the surge from the HT line. A SLOBLO will avoid this.

Your HT fuse is all about HT failure down stream of the fuse (ie a short in the power or preamp), not a rectifier fault, if that were the case your mains fuse would be failing.

neikeel wrote:I think you are on the right track.

It is probably a good idea to switch the HT at the ac (ie take your ac from the PT to a DPST standby switch, pull your bias feed off this if your dedicated bias feed is not up to the job.

- may I suggest that you hook the mains filter cap up directly to the +ve of the recifier and the ground of the rectifier directly to to the cap ground - which goes to its own dedicated chassis ground lug.

- then put your HT fuse in line taking the first lead of your choke and HT out to the board B+ line

As you are if you are using a fast blow fuse you may be hitting it with the surge from the HT line. A SLOBLO will avoid this.

Your HT fuse is all about HT failure down stream of the fuse (ie a short in the power or preamp), not a rectifier fault, if that were the case your mains fuse would be failing.

Thanks for the feedback!

Let me run this by you: The DPST switch would get the transformer HT lines, then bounce new HT lines from the other side of the switch down to the rectifier. Then I would take one side of the HT line (hot all the time or switched?) down to the bias circuit. (BTW, I did that already down at the rectifier now but the picture as of 06/17/15 is old and doesn't show that.)

This arrangement would mean that HT is running to the switch when power is turned on, but only when standby switch is flicked, then sends the HT vac to the rectifier. EDIT: Which switch would you use? http://valvestorm.com/Products/Switches

After the rectifier, you suggest taking the +ve out of the rectifier to the first side of the filter cap, then to the second side of the filter cap, then to the HT fuse, then from the other side of the HT fuse to the B+ lug at the board. The -ve would go to the negative lug of the filter cap which is (currently) grounded to the chassis. I had wondered if the +ve was so raw, it was slamming the fuse too hard. I was reticent to put in a SLOBLO fuse in there just in case.

The overall affect of this is keeping the HT from the rectifier until standby is switched. When the standby is flipped, HT vac gets rectified, filtered then hits the HT fuse which is sent to the B+ lug.

I don't have the DPST switch available, so I am still using the SPST switch. Hopefully this isn't too dangerous.

Rectifier +ve to one side of the standby switch. Then the other side of the standby switch goes to the 50uf cap in the can, to the other 50uf cap in the can then to the HT fuse. From the HT fuse to the B+ lug where the OT center tap and one leg of the choke. The above with taking the negative of the bridge rectifier to the negative side of the 50/50uf cap can, I am now getting 467vdc!!!

I need to clean up the wiring, but hot dog! Here are my voltages.



Bias circuit needs attention, clearly, but with preamp tubes in. All look good!

Good, and yes that is why you wanted the can in circuit direct off the rectifier (if you cannot use the DPST).

neikeel wrote:Good, and yes that is why you wanted the can in circuit direct off the rectifier (if you cannot use the DPST).

I can't thank you enough. I am probably going to pull out the bias tap and hook that up again. It's just easier and I read about some benefit to using a dedicated bias tap in that impedances are matched better on the HT supply.

Any ideas why I would have a 10v difference between the two pin 5 measurements on the EL34s?

marshallnoise wrote:

neikeel wrote:Good, and yes that is why you wanted the can in circuit direct off the rectifier (if you cannot use the DPST).

I can't thank you enough. I am probably going to pull out the bias tap and hook that up again. It's just easier and I read about some benefit to using a dedicated bias tap in that impedances are matched better on the HT supply.

Any ideas why I would have a 10v difference between the two pin 5 measurements on the EL34s?

Check the grid resistor values and your solder joints, are your PI couplers in good order, that is all it could be (unless weirdly defective EL34 but I guess you flipped them to see if it follows tune or socket?

Actually, I haven't plugged the EL34s in yet. I was trying to get a negative DC voltage reading on pin 5 in the neighborhood of -37 before plugging them in. All the other readings posted above were done with 12AX7s in. I need to read up on the preliminary setup of the bias some more.

You expect between -30 and -50v at the junction of the two 220k grid resistors on the board (check the orientation of your bias diode) and the value of the dropping resistors as well as the orientation of your bias caps (+ to ground).

What voltage does your bias tap give at the voltages you report above? A 100w has 95vac before a 27k bias feed, throughout the diode and 15k decoupler between the bias caps. The 50w obviously uses half the ac through a 220k (a JTM45 a 180k bias feed).

neikeel wrote:You expect between -30 and -50v at the junction of the two 220k grid resistors on the board (check the orientation of your bias diode) and the value of the dropping resistors as well as the orientation of your bias caps (+ to ground).

What voltage does your bias tap give at the voltages you report above? A 100w has 95vac before a 27k bias feed, throughout the diode and 15k decoupler between the bias caps. The 50w obviously uses half the ac through a 220k (a JTM45 a 180k bias feed).

It'll be later tonight but I will get it metered out.

I haven't reinstalled the stand alone bias supply yet. I also need to reground the speaker jack. I had originally thought that grounding the speaker jack was contributing to me popping the HT fuse.

Here are the updated pics. FYI, if you click on the pictures, you get the larger size and the full scale.