Coupling Diode between HT Plate and HT Screen Supplies

[nevetslab]

I'm now in the process of checking out this cobbled Hiwatt DR201 Bass Amp circuit built into a discarded Blackstar S200 Chassis. Unloaded, the HT Plate circuit is 730VDC, and the HT Screen circuit is 402VDC. I have the standby switch where it's shown in the provided Hiwatt DR201 Schematic I drew up a couple months ago. When I switch out of Standby, the charging of the filters in the Preamp circuit drops the Screen Supply down to 310V, where it rapidly charges back up to around 390VDC....takes about 15 sec to get to 390V, while it's up at 370V within a couple seconds.

Now switching back to Standby, the HT Plate supply drops immediately, as there are no additional supply caps to hold it up. The Screen Supply takes a good deal of time to drop to near 0V....like 30sec to 200V, a minute to 100V, 2 min to get to 30V, etc.

Hiwatt DR201 Bass Amp Schematic-2B.pdf

I've seen a 1N4007 or equivalent rectifier diode connected between the Screen and HT Plate supplies....Anode to Screen, Cathode to Plate on some circuits using 6550's or KT88's. I didn't see one used in the DR201 I had here in January, and didn't see one in any of the Hiwatt 200W amps built with the dual supplies like this. I gather its' purpose is to prevent screen voltage from being present on the tubes when there's no Plate Voltage, which would be the case when one switches a Hiwatt DR201 to Standby from Operate.

Is there potential for damage to the power tubes if there isn't this diode installed?

[Helmholtz]

With zero plate voltage, the powered screen acts as anode attracting all of the electrons. This could cause excessive screen current and dissipation.
I would watch for screens glowing red.

A diode that ties the screen supply to the falling plate supply looks like a good idea.

[Chuck H]

Agree with Helmholtz utterly here. Because I've seen it on my bench while dialing in an unusual design. I ended up going a different direction in the end but I can say that the screen needs to track the plate voltage with this implementation or it can become the primary current source.

[g1]

Depending on the purpose of the standby, it can sometimes be better to kill the HV to the screens instead of the plates.

[nevetslab]

Depending on the purpose of the standby, it can sometimes be better to kill the HV to the screens instead of the plates.

I noticed that method in a circuit Jon Wilder had put forth after assistance from R.G. in the Official Dual Rail Thread:



In this diagram, the HT Plate circuit stays up, while the S/B Switch is turning off the screen supply. Also note the reverse-bias rectifier diode that would follow the plate supply, should it's fuse on the output of the supply fail.

Thanks.

[Jon Snell]

I'm now in the process of checking out this cobbled Hiwatt DR201 Bass Amp circuit built into a discarded Blackstar S200 Chassis. Unloaded, the HT Plate circuit is 730VDC, and the HT Screen circuit is 402VDC. I have the standby switch where it's shown in the provided Hiwatt DR201 Schematic I drew up a couple months ago. When I switch out of Standby, the charging of the filters in the Preamp circuit drops the Screen Supply down to 310V, where it rapidly charges back up to around 390VDC....takes about 15 sec to get to 390V, while it's up at 370V within a couple seconds.

Now switching back to Standby, the HT Plate supply drops immediately, as there are no additional supply caps to hold it up. The Screen Supply takes a good deal of time to drop to near 0V....like 30sec to 200V, a minute to 100V, 2 min to get to 30V, etc.

[ATTACH]n964860[/ATTACH]

I've seen a 1N4007 or equivalent rectifier diode connected between the Screen and HT Plate supplies....Anode to Screen, Cathode to Plate on some circuits using 6550's or KT88's. I didn't see one used in the DR201 I had here in January, and didn't see one in any of the Hiwatt 200W amps built with the dual supplies like this. I gather its' purpose is to prevent screen voltage from being present on the tubes when there's no Plate Voltage, which would be the case when one switches a Hiwatt DR201 to Standby from Operate.

Is there potential for damage to the power tubes if there isn't this diode installed?

In standby there is no HT nor screen voltage. It uses a double pole toggle switch and does not require an extra diode, that is why we have never used a diode in that position.
I am curious as to why your diagramme calls for 120v neon and 500v neon. Neons flash over at around 100volts and hold the voltage at around 90volts, depending upon the mix of gasses. Neons on 120volts use a 82k series resistor and for 240volts 150k so probably 270k if on 500volts DC.

[nevetslab]

In standby there is no HT nor screen voltage. It uses a double pole toggle switch and does not require an extra diode, that is why we have never used a diode in that position.
I am curious as to why your diagramme calls for 120v neon and 500v neon. Neons flash over at around 100volts and hold the voltage at around 90volts, depending upon the mix of gasses. Neons on 120volts use a 82k series resistor and for 240volts 150k so probably 270k if on 500volts DC.

While the HT Plate has been switched off, there still IS Screen Voltage. It's potential is that from the filters on the preamp, which is in the state of discharge, but remained up for a couple minutes. I haven't yet placed the tubes into place to see what the difference is, but that was the concern.

The Multisim 11 program didn't have any other symbols for representing the two panel lamps Hiwatt uses. i don't know if I have any other photos that show the two lamps they use

[Jon Snell]

If there is any voltage in the lower HT supply when the amp is switched to standby, the screen grids of the output valves together with the preamp and phase splitter will deplete any voltage very quickly. HT2 only has 32u through a 10k resistor and a further decoupling 16u through another 10k resistor.
Where do you think any screen grid damage is going to come from? Answer, there is no likleyhood of any damage long or even short lived as the KT88 screen grids can happily dissipate 8Watts each all the time. That is why we did not fit clamp diodes ... it is unnecessary.

Your schematic is incorrect, unstable V1 to V4. Noted below.


Edit; Fit them if you want but they are not needed at all. Measure HT2 at turn off and you will see what happens.

[mhuss]

I agree with Jon, the KT88 screens should be able to handle it for a short time. Plus you have 1k current-limiting resistors in series with each screen as extra protection.

I don't think I've ever worked on a KT88 DR201 that had power tube screen issues.

[Helmholtz]

If there's no "redscreening", there's no problem.

OTOH, a discharge diode is cheap and won't hurt.

[nevetslab]

If there is any voltage in the lower HT supply when the amp is switched to standby, the screen grids of the output valves together with the preamp and phase splitter will deplete any voltage very quickly. HT2 only has 32u through a 10k resistor and a further decoupling 16u through another 10k resistor.
Where do you think any screen grid damage is going to come from? Answer, there is no likleyhood of any damage long or even short lived as the KT88 screen grids can happily dissipate 8Watts each all the time. That is why we did not fit clamp diodes ... it is unnecessary.

Your schematic is incorrect, unstable V1 to V4. Noted below.


Edit; Fit them if you want but they are not needed at all. Measure HT2 at turn off and you will see what happens.

The schematic I composed is from working thru the older DR201 schematics along with the photos and layout of the current production DR201 that was here over the holidays end of last year.



The bottom half of the HT Plate Caps is the first buss cap on the right, first photo. Both 100uf/500V sections in parallel. The Top side of the HT Plate is across the chassis, the single 220uf/500V cap. Then, the second section of the 400V Screen Supply is that following the 100 ohm/7W decoupling resistor. The third filter section following the 1k/7W decoupling resistor at the end of the Preamp Turret Board is the last dual 100uF/500V cap on the left photo. The last two filter sections are 50uF/500V each, that located on the input side of the Chassis.

Following the layout diagrams I had from the DR504, and the other variations of the DR201 and one other, I worked out the wiring diagram on the bottom of the Preamp Turret Board:



The schematic I drew was based on earlier versions of the DR201 and this working amp, and taking the time to work out the unknowns until it all made sense (to me, at least). I've used this Preamp Turret Board layout on what I built in the rendition of a DR201 in this Blackstar chassis, and this morning, with tubes installed (only one pair of 6550's for the moment), it's a functioning amp. I've made some kind of mistake on the wiring of the bass control, yet to be worked out, but apart from that, all is making sense.

The V4 tube stages are fed from the final decoupled P/S stage that feeds the Input Stage, in the diagrams I have that's common to the DR504, DR405, DR103, DR201.



This morning, I installed a pair of 6550's and biased them up to 20mA each, have the preamp and driver tubes installed, powered it up and down to watch the loaded change in the discharge rate from what I was seeing without the tubes installed. And, granted, the discharge rate will increase slightly with four power tubes installed. The HT Plate goes to zero right away, of course, while the Screens are held up by the remaining charges of the filter caps in the Preamp stages. It drops from 374VDC to 1/10 that in 30 seconds, so I agree with everyone that there's no danger here of damaging the power tubes for that short duration with no plate potential during standby drain.

Thus far, the amp passes signal ok, no oscillation for the brief period I've looked at it. Just at the starting phase of inspection. Late yesterday, I had moved the Standby Switch to couple the HT Screen Supply to the screens, and tied the 220uF/450V buss cap sitting at the end of the Power tube Turret Board to the 1st filter stage, so it agrees with the Hiwatt DR201 schematic I drew up.

I looked at what photos I had of the Power and Standby lamps that sit above the Power and S/B Switches on the DR201 chassis. But, the lamps are wrapped inside heat shrink, and all I have are the wires exiting those assemblies. So, I don't have any details. The indicator lamps I placed on the schematic are, for the time being....place holders until I come up with something else that represents what they have.

While I have a working relationship with Hiwatt for doing their Service in the LA Area, they offer no tech support as far as documentation, details and such that one would expect from a manufacturer in a relationship like this. Building this DR201 model in a Blackstar chassis was intended as a working Test Fixture to screen and match KT88 & 6550 Power Tubes for all the amps I work on for CenterStaging and other clients while access to affordable production power tubes has been in crisis from Putin's desire to recreate the Soviet Union as it once had been before any of us were born., including him.

So, I appear to be getting close to a functioning unit.

[Jon Snell]

You have made a good job of building it. Good luck with getting it fully functional and I don't think you will have any trouble with that.

An excellent project.

[nevetslab]

This morning, having recovered from some wiring errors in getting the HT GND Fuse wired in correctly (I didn't have the (-) side of the two Bridge Rectifier circuits wired in correctly, so the HT Fuse wasn't functional) late yesterday evening, I powered it up and got the latest assembly photos. Then connected my 4 ohm test speaker, and plugged in pink noise to give a listen. Passes signal ok, both input channels work, but the tone circuits are NOT working as they should. Bass boost works, but turning fully CCW, I loose signal almost completely. That 500k Audio Pot measures more like 250k Audio. Removed it from the panel, saw it labeled correctly, though I don't recall if I had checked it prior to installation. I'm not getting any response from the Treble pot nor from the Midrange pot. Presence control appears to work. I did substitute 220k's for the 470k Grid-wiper resistors from the two Volume pots, not having any 470k 1W resistors yet.

So, I may NOT have the wiring correct for the Preamp Turret Board. I have had no assistance from Hiwatt's Tech up in Toronto, he being the only one there who has any smarts that I'm aware of. I've not received ANY layout wiring diagrams of the DR201 Preamp Turret Board, so it could be the error is from my presumptions in creating one from the DR201 photos, schematics and of the layout drawings I did get from him for the Custom 50/100W amps. Maddening not having proper Tech Support when being their tech in the LA Area!

So, I'm starting from the assumption my layout photos are in error (having used colored lines to connect each component on the Turret Board to all of the parts on the board, as well as to the Preamp Tubes and panel controls), correcting it as I progressed until I had what appeared to agree with the schematic. I then used THAT as the Preamp Turret Board guide to route the wiring on the bottom of my Turret Board and then connecting to the Panel Controls and Tube sockets. As it's all passing signal, and from having added an input jack at the top of the Master Volume so I can bypass the Front end, and go to the Power Amp section, all that works fine as well, I'm not too far off. Hopefully it's not a major routing error on the bottom of the Turret Board. Getting that board back out at this stage isn't anything I would hope for.





The rear panel Status Lamps, HT Plate and HT Screen/Preamp test points are reading properly with their 1000:1 dividers, the Bias Voltage Adjustment pots working fine, as are the test points for measuring current thru the power tubes at the four 1 ohm cathode resistors on the rear panel. When a Cathode HT Fuse opens, I see about 11VDC from Ground to the top of the Cathode Resistor, so the 22k source resistor tied there to power the LED to light it if there's a fault works, and is within safe limits for the power tubes. AC Mains Idle current with 20mA cathode current flowing thru the four power tubes is shown (on the power analyzer) in the second to last photo, while Standby Mains Current shown in the last photo. HT Screen Voltage is seen still discharging on the DMM, plugged into the HT Screen Test Point. So, the objectives as far as using this as a Test Fixture appears to be progressing.

I still have to pull the Cabinet out, lay in a sheet of Aluminum on the floor of the cabinet, Cut an opening for the 120mm Fan to pump air thru the end vents to cool the heat sink, as well as direct heat away from the power tubes above the top of the chassis, and also open relief vent on the opposite side of the cabinet from the heated exhaust from the insides of the chassis. Then gotta deal with the handles. Blackstar's decision to place a single handle on the top middle of the cabinet, when all of the weight is at one end is quite amusing, as well as stupid.

Onward...........

[nevetslab]

Last night, I thought I had found one of the EQ caps being the wrong value. I did find a mistake in values on the DR201 Schematic....C6 across the Midrange Pot was shown as 22nF, while it being 1000pF on the build, and was also that value on the Hiwatt DR201 build that was here, which I had photos of.

This morning, I continued my reconstruction of the Layout on the bottom side of the Preamp Turret Board, and verified everything...except I was still getting the Bass Control to cut the signal out when turning the pot fully CCW. Only way that could happen would be if the Plate of V2 Pin 6 was NOT at R12/R14, but instead at R14/C8/C7. That's where I found the connection. DUH?!!! So, I managed to carefully lift the socket side of the PCB up, after removing the two mtg screws, unsoldered that BLU plate wire from R14 and moved it over one to R12 where it was supposed to be. Re-soldered everything and put the board back down.



Set the tools aside, re-connected AC mains, 4 ohm test speaker, Burst Pink Noise, and powered back up.

I now have a working Hiwatt DR201 Clone. I haven't yet plugged my bass into it, but just listening to it, it sounds nice and quiet. Easy mistake to make....both 100k resistors being next to each other.

I watched the current readings thru the four Power Tubes' cathode resistors under drive, watched the power supplies fluctuate under drive, along with the AC Mains current/wattage during bursts. So, this is doing what it should, as well as providing for testing KT88's and 6550's, as well as testing/sorting thru ECC83/12AX7 preamp tubes.

I haven't yet made any qualitative measurements on the amp....including listening for RF buzz from the preamp tubes, as well as hum. In fact, I haven't yet listened to what the power transformers are coupling into the system, using the O/T to pick that up. Easy to do.....disconnect the HT supplies and see what it finds, with our without HT supplies connected, but not connecting the O/T primary C/T.

Onward.........

I did find after calculating the value of the 500k Bass Control in the wiring with the rest of the pots and resistors that would load the Bass control, that the value I was measuring WAS correct.. With the Bass pot turned all one way, with the Mid and Treble Pots turned the opposite way, I saw it was around 170k, and going in the other direction with all three controls, it was around 280k. That agreed with the measurements I was getting with the ohmmeter across the bass control. So, it was indeed a 500k pot. Just being loaded by all the other controls. Unsoldered the wire that strapped the wiper to the top of the pot, it measured 490k, just to confirm it.