Direct- vs. indirect-heated rectifiers in ST120

As I understand, a direct-heated rectifier applies high voltage to driver and power tubes faster than an indirect-heated rectifier. But what I read suggests that a direct-heated rectifier takes 3-4 seconds to heat which I think is about equal to the Weber Copper top SS rectifiers which I have used in the past in my ST120. So, my question is can a 5 V 3 A tube rectifier be used in the ST120 without undue stress on the tubes? As a side question, it appears direct-heated rectifiers have 4 pins while an indirect-heated rectifier will have 5 pins. Is this correct?

eickmewg wrote:As I understand, a direct-heated rectifier applies high voltage to driver and power tubes faster than an indirect-heated rectifier.  But what I read suggests that a direct-heated rectifier takes 3-4 seconds to heat which I think is about equal to the Weber Copper top SS rectifiers which I have used in the past in my ST120.  So, my question is can a 5 V 3 A tube rectifier be used in the ST120 without undue stress on the tubes?  As a side question, it appears direct-heated rectifiers have 4 pins while an indirect-heated rectifier will have 5 pins.  Is this correct?

No rectifier , tube or solidstate, will stress the tubes.

What might be of concern is that if a rectifiers is active before the powertubes start conducting
B+ will be higher then in normal use. If the caps rating is close to normal use then
they might have overvoltage during the time until the powertubes start conducting.

The 5V3A will work just fine in the ST120. It will be much more reliable than a 5AR4. You'll likely have no trouble with it for many years. You'll have slightly lower B+, but unless you are trying to rattle your neighbor's windows, you'll never notice.

I asked Bob L. awhile back about the exact same issue that peterh mentioned about the capacitors. Bob said that he has not had a single Cap failure from any of his customers and that the Caps can take the voltage.

This is good to know. I always learn something new when I ask a simple question. I was unaware the primary issue was with the cap voltage surge on startup. I was looking at a 5Z3P rectifier but I think that tube won't fit into a standard octal socket. I'll look into the 5V3A. Thanks all for your help.

I use a 5V3A in a ST120 and now have several hundred hours on it. Plate voltage is between 445-450 volts depending on line voltage. I've picked up a handful of spares on eBay but haven't used one yet. Bias hasn't been adjusted in months.....

Well , I had a 5U3C Winged C NOS, made in CCCP on hand. The accompanying leaflet is unreadable (Russian) but underneath states "20000 25-XI-70. This suggests a batch of 20.000 in 1970.....

Anyhow, I finally mounted it in my ST-120, it biased perfectly and is now playing as good (or better??) as always.

The bottle is a bit shorter and thicker than Skizo's favourite Mullard GZ37 (that I also have on hand.)

Those rectifiers really keep us going...... Must admit that I like tube over Solid State rectifiers, if alone for the looks and the glow of it..

Good to know yet another option verified by a happy user.

The 5U3C is way more affordable than the big bottle mullards too. I see the winged C going for around $50 ... afraid to see what a NOS GZ37 goes for nowadays. Glad I got spares when I did!

PS ... as usual, the Cyrillic alphabet can get confusing. The Russian 5U3C is actually a 5U4-G ... Same thing, only different, as they say.

Well Skizo, I've just ordered two more Svetlana Winged C NOS 5U3C (5u4-g) for 16 € (19$) each at TAD (Tube Amp Doctor)
The amp seems to like it........and it will probably outlast a whole box of GZ34's

My GZ37 was well over 130$....... including VAT and shipment , so I keep it for "special occasions"
(http://www.watfordvalves.com/search.asp?search=GZ37&Submit=Search)==> 72 £ excl VAT and shipment.

Are you folks eating rectifiers ???

I had the chinese 5ar4 that bob sent, it broke in a power surge, since then i have used a JJ 5ar4
for 1 1/2 year. This is not a st120 but a vta70. I do however have a "power sequencer" that
will make sure that no hot-switching occurs. In total 1 5ar4 in 2 1/2 years. ( 2335h powered on)

If the 5ar4 breaks, i will install 2 1n4007 , at least that's my plan.

peterh wrote:Are you folks eating rectifiers ???

I had the chinese 5ar4 that bob sent, it broke in a power surge, since then i have used a JJ 5ar4
for 1 1/2 year.  This is not a st120 but a vta70.  I do however have a "power sequencer" that
will make sure that no hot-switching occurs.   In total 1 5ar4 in 2 1/2 years. ( 2335h powered on)

If the 5ar4 breaks, i will install 2 1n4007 , at least that's my plan.

Lucky! I never had a newer rectifier tube last more than a year and on 2 occasions not more than 1 day. Never again! I built my ST120 with UF4007's and never looked back.

Based on this thread, I have snagged a supposedly NOS Svetlana Winged =C= 5U4G rectifier off the bay for $14.98 shipped. It was the only tube I found in the US, all the others were coming from Eastern Europe. I will be pleased if it outlasts a box-full of GZ34/5AR4 rectifiers. I also tried some 5Z3PAT tubes from China but they may not be octal-socket tubes. We shall see.

eickmewg wrote:Based on this thread, I have snagged a supposedly NOS Svetlana Winged =C= 5U4G rectifier off the bay for $14.98 shipped.  It was the only tube I found in the US, all the others were coming from Eastern Europe.  I will be pleased if it outlasts a box-full of GZ34/5AR4 rectifiers.  I also tried some 5Z3PAT tubes from China but they may not be octal-socket tubes.  We shall see.

The 5Z3PAT is an octal socket tube. I have 2 of them. It acts just like a 5U4.

Just about any rectifier tube will last longer than a 5AR4.