grounding issue with "Pro-ject tube box se ii" phono stage

[HiMcDunnough]

hi all

Here's the setup that I'm having problems with
Pro-ject phono stage (12ax7a)
Lector ZOE preamp (no phono stage), (Telefunken 12au7 tubes)
Adcom GSA-555.

- I've been using a Cambridge 651p (budget ss phono stage) with zero problems.
- the pro-ject works as it should for five minutes or so, after which I get a low hum, like a grounding issue.
-I've disconnected and reconnected it several times, taking care to ensure the ground connection from TT to the phono stage, but with the same results each time.
-switching back to the Cambridge resolves the issue

-Since I don't have grounding issues at all with the other phono stage I'm wondering if it could be something tube related. Is 12Ax7a to 12AU7 a bad match?

Any insights? Any help would be greatly appreciated

Thanks for reading

 

[HiMcDunnough]

Phono stage specs:
output gain 60 dB
output 300mv at 1kHz for 3mv 0,3mV

Lector preamp specs:
Input sensibility and gain: 200 mv / 20 dB gain
output: 20 Volts @250 ohms

... looking at these specs, is the phono stage way too much power for the Lector's input levels

 

[century tek]

It's not the specs. You have a bad tube or a bad unit. 12AX7 and 12AU7 are not interchangeable. Pinout is the same but gain is way different between the two.

 

[HiMcDunnough]

Cool
so it would be worth it to try some new tubes in the pro-ject. I have them available and will try that. I wasn't thinking 12AX7 tubes are interchangeable, but I was confused as to whether the 12AX7s in that particular phono stage were a bad match with my preamp which uses 12AU7 tubes (which is maybe just as dumb, not sure .)

Thanks

 

[HiMcDunnough]

Well, I swapped the stock tubes with some choice telefunkens and got the same result. Unless I hear otherwise, I'll assume this pro-ject phono stage is beneath me and move on

 

[HiMcDunnough]

I thought I'd update this: I swapped out my cartridge and the hum vanished... I don't know what to say really. If anyone's got ideas as to why my cartridge, that was silent using a solid state phono stage, produced a loud hum using a tube phono stage, I'd be happy to hear it. Thanks

 

[bberkom]

Being a tube preamp, it could be more sensitive to other types of interference than the ss preamp. The cartridge is basically a pickup.

Perhaps there was corrosion or some other issue with the tonearm leads, headshell contacts, or RCA's producing a bad ground when you installed the Pro-ject phono preamp. When you swapped the cartridge, maybe you managed to deal with the poor connection. I would also make sure the shields of the RCA cables connecting the turntable and phono pre are clean and fitting tight enough since it sounded like a grounding hum.

 

[HiMcDunnough]

I'm happy if its that the tube pre is just more sensitive and needs a better cart...
It definitely sounds like a ground hum. I double checked the cables, but every time I put in the AT cart - buzz city - .