Citation V McShane Mod Questions

[Reek]

Hello everyone,

I'm working on my first Harman Kardon Citation V using the complete McShane rebuild kit. I've emailed Jim, but I know he's busy, so I thought I'd ask those with firsthand experience.

1. Feedback Capacitor Values​

My amplifier originally has:

• C10/C16 = 47 pF
• C6/C12 = 220 pF

The McShane kit includes:

• 100 pF
• 430 pF

The installation notes mention contacting Jim if the original values are preferred, but don't explain why.

What are the advantages and disadvantages of keeping the original capacitor values versus installing the larger values? I'm assuming this has to do with amplifier stability and HF compensation, but I'd appreciate hearing the reasoning from those who've done this rebuild.

2. 12BY7A 1K Resistors​

My amplifier does not have the 1K resistors associated with the 12BY7A tubes.

I've read another thread suggesting these were added to compensate for today's higher line voltages (115–125 VAC instead of the original 110 VAC), reducing the heater voltage from around 6.3 V to approximately 5 V.

Should these resistors be added if they aren't already present, or is it better to leave the heater circuit original?

3. Choke Upgrade / C4 Wiring​

My kit includes the choke upgrade, which installs where C4 is located.

The notes simply state that C4 is not replaced, but they don't explain what should be done with the wiring that originally went to C4.

Also, the McShane Citation V power supply schematic with the choke still shows C4A and C4B connected in parallel with the secondary winding feeding the DC balance pots.

I'm having trouble reconciling the schematic with the physical wiring.

Can someone who has completed the choke conversion explain exactly what happens to the original C4 connections and how they should be rewired? Photos would be greatly appreciated.

Thanks in advance. I'm an experienced electronics technician, but this is my first Citation V restoration, and I'd rather ask questions than make assumptions.

 

[nerdorama]

The choke replaces R45, the 270 ohm power resistor. The choke should be one with 270 ohms resistance. Can't remember the inductance rating. The schematic I used shows 100p and 430p. Maybe yours is an older version.


Not sure what the 1k resistors are. The heaters should have 1 ohm resistors in series to drop the voltage.

 

[ramjet]

Hello everyone,

I'm working on my first Harman Kardon Citation V using the complete McShane rebuild kit. I've emailed Jim, but I know he's busy, so I thought I'd ask those with firsthand experience.

1. Feedback Capacitor Values​

My amplifier originally has:

• C10/C16 = 47 pF
• C6/C12 = 220 pF

The McShane kit includes:

• 100 pF
• 430 pF

The installation notes mention contacting Jim if the original values are preferred, but don't explain why.

What are the advantages and disadvantages of keeping the original capacitor values versus installing the larger values? I'm assuming this has to do with amplifier stability and HF compensation, but I'd appreciate hearing the reasoning from those who've done this rebuild.

2. 12BY7A 1K Resistors​

My amplifier does not have the 1K resistors associated with the 12BY7A tubes.

I've read another thread suggesting these were added to compensate for today's higher line voltages (115–125 VAC instead of the original 110 VAC), reducing the heater voltage from around 6.3 V to approximately 5 V.

Should these resistors be added if they aren't already present, or is it better to leave the heater circuit original?

3. Choke Upgrade / C4 Wiring​

My kit includes the choke upgrade, which installs where C4 is located.

The notes simply state that C4 is not replaced, but they don't explain what should be done with the wiring that originally went to C4.

Also, the McShane Citation V power supply schematic with the choke still shows C4A and C4B connected in parallel with the secondary winding feeding the DC balance pots.

I'm having trouble reconciling the schematic with the physical wiring.

Can someone who has completed the choke conversion explain exactly what happens to the original C4 connections and how they should be rewired? Photos would be greatly appreciated.

Thanks in advance. I'm an experienced electronics technician, but this is my first Citation V restoration, and I'd rather ask questions than make assumptions.

1. If you are using the SAMS schematic to do ANY work on the Citation V you should stop, it is full of errors. Throw it away. Use this HK Citation V schematic?
2. The feedback circuit could have been a 10k resistor with a 100 pf capacitor or a 12k resistor with a 47pf capacitor. I used the later values in both of the Citation V restorations I did this year and they are stable, but you also must perform item 5 below.
3. Use the 1 ohm resistors (not 1k) in the 12BY7A filament supply. These were used in at least one of the original versions.
4. As previously posted, the choke replaces R45 (off the real schematic), a 270 ohm 10 watt resistor. C4 is in the bias supply.
5. C7 and C14 (real schematic) were .47uf 400vdc capacitors. Replace these with 1uf 400vdc capacitors. This is a Jim McShane suggestion and I can vouch that this is necessary for low frequency stability.
6. Be very careful on your output tube selection. True 7581 or 6L6GC tubes are to be used unless you have the ability to lower the output tube current through bias.
7. Install a thermistor in the ac input; it will lower the ac slightly and stop the turn on thump. A CL60A will do.

 

[Reek]

1. If you are using the SAMS schematic to do ANY work on the Citation V you should stop, it is full of errors. Throw it away. Use this HK Citation V schematic?
2. The feedback circuit could have been a 10k resistor with a 100 pf capacitor or a 12k resistor with a 47pf capacitor. I used the later values in both of the Citation V restorations I did this year and they are stable, but you also must perform item 5 below.
3. Use the 1 ohm resistors (not 1k) in the 12BY7A filament supply. These were used in at least one of the original versions.
4. As previously posted, the choke replaces R45 (off the real schematic), a 270 ohm 10 watt resistor. C4 is in the bias supply.
5. C7 and C14 (real schematic) were .47uf 400vdc capacitors. Replace these with 1uf 400vdc capacitors. This is a Jim McShane suggestion and I can vouch that this is necessary for low frequency stability.
6. Be very careful on your output tube selection. True 7581 or 6L6GC tubes are to be used unless you have the ability to lower the output tube current through bias.
7. Install a thermistor in the ac input; it will lower the ac slightly and stop the turn on thump. A CL60A will do.

Thanks for the link to the orig. schematic. item 3. I understand and (oops) correct 1ohm not 1k.
my question regarding item 4. the McShane instructions says "you won't be replacing the C4A/b bias cap, you'll be using that spot to mount the new choke". I must be missing something that is probably staring me right in the face.. what happens with C4A/B? just physically relocated with new (smaller) caps and kept in circuit? thanks in advance for additional guidance.- Rick

 

[ramjet]

Thanks for the link to the orig. schematic. item 3. I understand and (oops) correct 1ohm not 1k.
my question regarding item 4. the McShane instructions says "you won't be replacing the C4A/b bias cap, you'll be using that spot to mount the new choke". I must be missing something that is probably staring me right in the face.. what happens with C4A/B? just physically relocated with new (smaller) caps and kept in circuit? thanks in advance for additional guidance.- Rick

I did not use the McShane kit for either of the two Citation V amplifiers so I do not know how he implemented this change. He does answer email.

 

[TriodeLuvr]

I restored my Citation V using Jim's power supply components (including the optional screen choke) a few years ago. I'll try to answer your questions based on my experience. Note that the component designators you're using are from the SAMS publication. The HK factory schematic is different.

1. I didn't buy those particular caps from Jim, and I can't tell you why he supplies those values. The original feedback network in mine consisted of a 47pF in parallel with 15K. I changed the cap to a 68pF during bench tuning. The caps in the step networks are 220pF like yours, but I replaced them with polystyrene. 10kHz square waves are respectable with these values and the amp is stable.

2. The 12BY7A step network in mine had 3.3K resistors, not 1K as shown in the schematic. I left that value as found. The 1Ω filament resistors had been removed (along with the mini terminal strips) by a previous owner. I reinstalled those parts.

3. Here's a short writeup of my Citation V on AK. It includes a few pics of the inside after completing the work. If you need more than this (and if you're not in a hurry), I'll try to drag the amp out of storage over the next few days and provide additional details.

Citation V Performance

FWIW, I also added a three-wire line cord with a Heyco strain relief. I soldered the ground wire to the steel chassis on the inside. I also later lifted the power supply ground off the chassis to eliminate a ground loop hum that appeared when the amp was installed in my system. Signal and power supply grounds are now completely isolated from the chassis.

Additional notes:

Mine had sticky, greening wiring everywhere inside. Only the transformer leads were unaffected. This is the result of the wire manufacturer using a defective plasticizer in the PVC insulation. It breaks down and forms hydrochloric acid, causing the copper wire to turn green. It also releases a corrosive gas that can damage pots and other chassis components over time. I stripped out all the original wiring and replaced it with new wire from Antique Electronic Supply. The input coax was also sticky, so I replaced that also. This work consumed many hours, much more than the restoration process I had anticipated.

The 3.3Ω output tube cathode resistors had drifted way off value. I replaced them all with close tolerance parts.

The replacement choke from Jim is 10H 240Ω.

Because this amplifier is more than 50 years old, I replaced all the tube sockets. I was lucky to have exact replacements for all of them on the shelf. This also made the job of rewiring much easier. I just cut the wires connected to the sockets and tossed it all.

I installed 6P3S-E tubes in my amplifier because I had more than a hundred of them at the time. This tube easily handles the voltages, but dissipation is only about 25W (the 7581 is 30W). I set idle current to an appropriate value by adjusting a fixed resistor in the bias supply.

Hope this is helpful.

Jack

 

[ramjet]

I restored my Citation V using Jim's power supply components (including the optional screen choke) a few years ago. I'll try to answer your questions based on my experience. Note that the component designators you're using are from the SAMS publication. The HK factory schematic is different.

1. I didn't buy those particular caps from Jim, and I can't tell you why he supplies those values. The original feedback network in mine consisted of a 47pF in parallel with 15K. I changed the cap to a 68pF during bench tuning. The caps in the step networks are 220pF like yours, but I replaced them with polystyrene. 10kHz square waves are respectable with these values and the amp is stable.

2. The 12BY7A step network in mine had 3.3K resistors, not 1K as shown in the schematic. I left that value as found. The 1Ω filament resistors had been removed (along with the mini terminal strips) by a previous owner. I reinstalled those parts.

3. Here's a short writeup of my Citation V on AK. It includes a few pics of the inside after completing the work. If you need more than this (and if you're not in a hurry), I'll try to drag the amp out of storage over the next few days and provide additional details.

Citation V Performance

FWIW, I also added a three-wire line cord with a Heyco strain relief. I soldered the ground wire to the steel chassis on the inside. I also later lifted the power supply ground off the chassis to eliminate a ground loop hum that appeared when the amp was installed in my system. Signal and power supply grounds are now completely isolated from the chassis.

Additional notes:

Mine had sticky, greening wiring everywhere inside. Only the transformer leads were unaffected. This is the result of the wire manufacturer using a defective plasticizer in the PVC insulation. It breaks down and forms hydrochloric acid, causing the copper wire to turn green. It also releases a corrosive gas that can damage pots and other chassis components over time. I stripped out all the original wiring and replaced it with new wire from Antique Electronic Supply. The input coax was also sticky, so I replaced that also. This work consumed many hours, much more than the restoration process I had anticipated.

The 3.3Ω output tube cathode resistors had drifted way off value. I replaced them all with close tolerance parts.

The replacement choke from Jim is 10H 240Ω.

Because this amplifier is more than 50 years old, I replaced all the tube sockets. I was lucky to have exact replacements for all of them on the shelf. This also made the job of rewiring much easier. I just cut the wires connected to the sockets and tossed it all.

I installed 6P3S-E tubes in my amplifier because I had more than a hundred of them at the time. This tube easily handles the voltages, but dissipation is only about 25W (the 7581 is 30W). I set idle current to an appropriate value by adjusting a fixed resistor in the bias supply.

Hope this is helpful.

Jack

The green wire problem is one I failed to note in my reply. Must have been from the trauma of replacing ALL of the wiring. I replaced all of the tube sockets too. They are great sounding amplifiers when done but a real pain to restore correctly.

 

[gadget73]

At least it was a kit, so its possible to tear it down to nothing and re-build from the ground up with new wiring. Can't imagine its any fun but probably sucks less than doing that on something that is all riveted and doesn't have an assembly manual or wire harness assembly guide available.

 

[mcgjohn]

Hello everyone,



What are the advantages and disadvantages of keeping the original capacitor values versus installing the larger values? I'm assuming this has to do with amplifier stability and HF compensation, but I'd appreciate hearing the reasoning from those who've done this rebuild.

​

Welcome to AK.

Lots of great input on this. To specifically answer your question, changing those values in the feedback circuit will change the knee in the feedback frequency response. Years ago I rebuilt a CIT V but no longer remember the efforts to that detail level. Its possible Jim thought the slight change would improve stability. Really tough to say.