AC30 (TBX) stopped working.

[Pixel]

I think the T6.3A is a 3 amp fuse. The 5AR4 has a 1.9 amp heater.

[Helmholtz]

I think the T6.3A is a 3 amp fuse. The 5AR4 has a 1.9 amp heater.

There's no problem with the GZ34 heater circuit?

Schematic says F3 and F4 are T6.3 fuses, meaning 6.3A slow-blow.
Blown fuse was a T6.3.
A lower rating is even more likely to blow.

Cold heater filaments have a very low resistance, so there will be a turn-on surge current several times larger than nominal.
That's why it needs to be a quality slow-blow (time delayed) fuse of sufficient current rating.

[Chuck H]

There's no problem with the GZ34 heater circuit?

Schematic says F3 and F4 are T6.3 fuses, meaning 6.3A slow-blow.
Blown fuse was a T6.3.
A lower rating is even more likely to blow.

Cold heater filaments have a very low resistance, so there will be a turn-on surge current several times larger than nominal.
That's why it needs to be a quality slow-blow (time delayed) fuse of sufficient current rating.

Since F3 and F4 are reported to be 3A in the amps and the actual 6.3V filament current is about 4.8A I figured a 3A time delay fuse of each half of the winding seemed about right.?. I wasn't considering the current surge though. But at 6.3A off each half of the winding the circuit is fused at more than two and a half times it's current draw. I guess I don't see how that provides any safety measure.

EDIT: Also, since the actual fuses in the amp reviewed ar the Vox Showroom don't match the schematic AND the circuit voltage happens to be 6.3 I considered the possibility of a typo on the schematic. But it does seem that SC's amp has the 6.3A fuses, so yeah. Go with that then.

[Pixel]

I think the T6.3A is a 3 amp fuse. The 5AR4 has a 1.9 amp heater.

Yeah I messed up there, I was thinking the fuse was on the rectifier. Oops.

[Sea Chief]

There's no problem with the GZ34 heater circuit?

Schematic says F3 and F4 are T6.3 fuses, meaning 6.3A slow-blow.
Blown fuse was a T6.3.
A lower rating is even more likely to blow.

Cold heater filaments have a very low resistance, so there will be a turn-on surge current several times larger than nominal.
That's why it needs to be a quality slow-blow (time delayed) fuse of sufficient current rating.

Hi Helmholtz,

grateful for your input, as always. Cold.. do you think there's any relevance to the amp being very cold (I turned on at 1am, a cold night, & always a very cold room where the amp is) ? Or are you just using cold more generally, as in simply a turned-off amp.

I was in fact going to do a thread on keeping the amp in a cold room/ any issue? etc.

Thanks, SC

[Sea Chief]

Since F3 and F4 are reported to be 3A in the amps and the actual 6.3V filament current is about 4.8A I figured a 3A time delay fuse of each half of the winding seemed about right.?. I wasn't considering the current surge though. But at 6.3A off each half of the winding the circuit is fused at more than two and a half times it's current draw. I guess I don't see how that provides any safety measure.

EDIT: Also, since the actual fuses in the amp reviewed ar the Vox Showroom don't match the schematic AND the circuit voltage happens to be 6.3 I considered the possibility of a typo on the schematic. But it does seem that SC's amp has the 6.3A fuses, so yeah. Go with that then.

So am I going for a T6.3A then? I can't quite keep up!

The removed fuse was 6.3A, & screen-printed on the board next to the fuse 'socket' is 6.3A (for both F3 & F4). Surely the schematic then will correspond with 6.3A. I'll check.

Thanks, SC

[Chuck H]

Use 6.3A fuses for F3 and F4.

Buy a few in case you have a power tube problem. And I'll reitterate, install the fuses and plug the power tubes in one at a time. Waiting for each to warm up for twenty seconds before plugging in the next. If one has a short it will pop the fuse soon after being plugged in.

[Helmholtz]

But at 6.3A off each half of the winding the circuit is fused at more than two and a half times it's current draw. I guess I don't see how that provides any safety measure.

F3 and F4 are in series to the 6.3V winding, so both see the same (full) current.

[Chuck H]

F3 and F4 are in series to the 6.3V winding, so both see the same (full) current.

Thank you for the clarification. My perception was incorrect. Obviously I thought each half of the winding was carying half the current.

[Helmholtz]

So am I going for a T6.3A then?

Yes, and make sure they are slo-blo (time delay) types from a reputable manufacturer.
In this case I would prefer a European brand T6.3/250V. UL rated types are somewhat faster acting, so more likely to blow.

BTW, what is your loaded heater voltage (after replacing the fuse and with all tubes in)?

[Helmholtz]

Cold.. do you think there's any relevance to the amp being very cold (I turned on at 1am, a cold night, & always a very cold room where the amp is) ? Or are you just using cold more generally, as in simply a turned-off amp.

The latter.
Just meant before heat-up.
I don't think turning on at 0°C or even lower will make a difference as max. surge current is limited by the resistance of the heater winding.

[Sea Chief]

Use 6.3A fuses for F3 and F4.

Buy a few in case you have a power tube problem. And I'll reitterate, install the fuses and plug the power tubes in one at a time. Waiting for each to warm up for twenty seconds before plugging in the next. If one has a short it will pop the fuse soon after being plugged in.

Hi Chuck.

Great thanks for clarifying. 5x fuses bought/ will update midweek.

I've never put power tubes in whilst amp is on before- are we talking just the ON switch for this test so just the heater filaments ? Or fully Standby 'on' too ?

My knees are knocking a bit at the prospect. I might just try 1x first with all tubes in & see if the same thing happens.


SC

[pdf64]

I don't know that anyone reports failures with these amps relative to any use of the standby switch …

Just to note standby in these amps is notoriously awful.
The design is hot switching, but even worse is a manufacturing error in the initial few years production that has the rectified DC taken from the pin 2 heater, rather than pin 8 cathode (and heater).
So the hot switched surge current passes through the rectifier heater, resulting in it having a short life if standby is used.
if an amp has a GZ34 rectifier, the use of standby has absolutely no technical merit, and possibly significant disadvantages, especially so if it’s a hot switching type.

watching the yellow Standby lamp light on it's own, after 30 seconds or so, seems a bit odd to me

Its completely normal.
The GZ34 warms up and gradually ramps up the HT voltage. When it gets over 90V the neon standby bulb strikes, illuminates.
Ignoring the standby switch minimises stress at power up.

[Sea Chief]

Just to note standby in these amps is notoriously awful.
The design is hot switching, but even worse is a manufacturing error in the initial few years production that has the rectified DC taken from the pin 2 heater, rather than pin 8 cathode (and heater).
So the hot switched surge current passes through the rectifier heater, resulting in it having a short life if standby is used.
if an amp has a GZ34 rectifier, the use of standby has absolutely no technical merit, and possibly significant disadvantages, especially so if it’s a hot switching type.


Its completely normal.
The GZ34 warms up and gradually ramps up the HT voltage. When it gets over 90V the neon standby switch strikes, illuminates.
Ignoring the standby switch minimises stress at power up.

Hi there Pete.

Thanks this clarifies & gives me a bit of confidence to keep using it like you suggest, IE keep Standby in the 'on' position & forget it. Even if it 'kinda feels like' this way is putting undue stress on. Ok so in fact the very opposite using it like so.

Remind me, do you have one of these AC30/6 TB's? Or do you know them from fixing some maybe- you've obviously got a very comprehensive knowledge of the circuit, flaws & all.

Thanks, SC

[Chuck H]

Just to note standby in these amps is notoriously awful.
The design is hot switching, but even worse is a manufacturing error in the initial few years production that has the rectified DC taken from the pin 2 heater, rather than pin 8 cathode (and heater).
So the hot switched surge current passes through the rectifier heater, resulting in it having a short life if standby is used.
if an amp has a GZ34 rectifier, the use of standby has absolutely no technical merit, and possibly significant disadvantages, especially so if it’s a hot switching type.


Its completely normal.
The GZ34 warms up and gradually ramps up the HT voltage. When it gets over 90V the neon standby switch strikes, illuminates.
Ignoring the standby switch minimises stress at power up.

Well I've been all over the place mistaken about stuff on this thread! Glad you and Helmholtz were here. Since we're at it let's just put it all on the table. Is there a problem with my suggestion to plug in the power tubes one at a time? SC is nervous about it but it seems safe enough to me and a probable way to find out if a power tube is responsible for the blown fuse. There's the chance that as current increases with each added tube that a fuse may blow anyway. But even that might help diagnosis.