Cassette Player / Valve Amp 50EH5 DIY'ish

[DashApple]

Hey ,

This was my weekend project that I have nearly finished ( just need to do low voltage PSU, Power connector )

The overall goal was a open top cassette player and stereo valve amp all in one box .

short video can be found here of it running - https://www.youtube.com/watch?v=yWrDe6XFIik&list=UUvjDVNxRU2DInDMQQw_iYJA

The cassette player is from a dead portable unit with the motor driver being replaced by a very low dropout PMOS voltage regulator

The preamp for the tape head is based in a Philips TDA1522 IC and the board its on is mounted to the front of the unit .

The Valve amplifier is based on the 12AU7 preamp tube and 50EH5 pentode with negative feedback .

Most of the cable are screened with one end grounded to avoid ground loops ( except for preamp , power stage ) , The preamp stage is fed via one shielded cable and the power stage is fed from a dual core screened cable from the output transformers .

The power transformer was a custom one I had made to provide the 50V , 6.3V and 130V AC needed for the amp .

The tape preamp is run from 12V and the motor driver 3.7V ( power supply build in progress )

The unit dose have a AUX audio input with a DPDT switch for input selection ( Tape or external ) .

Worked out rather well : ) , No hum and good sound ( with proper speakers xD )

The inside , the black cable with the two red cables is the preamp side supply and the two red cables are the negative feedback to the preamp tubes cathodes ( other end is taken from speaker out jacks )

The other cable with the orange wire is the dual core from the output transformers and the orange cable is the supply to the output tube grid 2's

IMG_0863 by TwirlyWhirly555, on Flickr

The 3 electrolytic capacitors and the two polypropylene capacitors and 2 resistors make up the power supply stage

IMG_0867 by TwirlyWhirly555, on Flickr

IMG_0868 by TwirlyWhirly555, on Flickr

IMG_0869 by TwirlyWhirly555, on Flickr

IMG_0871 by TwirlyWhirly555, on Flickr

This is the top showing the preamp tube ( front ) and the two power output tubes , The cassette player raised and held in place with two bolts and 6 nuts , its cables run though a cutout in the from panel .

The front panel , the board in the middle is the tape head preamp , left side is the speaker jacks and the right side in input selection switch , volume pot and 1/4" input jack .

IMG_0874 by TwirlyWhirly555, on Flickr

 

[ped]

Tremendous work.

I haven't seen axial electrolytics for a long time!

Ped

 

[DashApple]

Tremendous work.

I haven't seen axial electrolytics for a long time!

Ped

Thanks ! : D

They are quit popular in valve based builds I believe , it makes point to point wiring a lot easier

 

[DashApple]

Well its finished , replaced wire with shielded PTFE cable , Low voltage PSU done .

1.6Va toroidal transformer ..... Kinda cute ... : P

IMG_0885[1] by TwirlyWhirly555, on Flickr

IMG_0887[1] by TwirlyWhirly555, on Flickr

 

[Sigurthr]

Awesome, awesome, awesome build!

What negative feedback scheme are you using? Any frequency filtering or straight resistive feedback?

What output topology are you using? Output transformers, capacitive anode take off, push/pull? It looks like you're using output transformers but those could also be interstage/impedance match between the 12AU7 and the final pentode.

You're supplying the filaments with AC right? Amazing that there's no trace of hum! I have a small 12AU7 headphone amp, but it uses DC on the filament to avoid 60hz in the output.

 

[DashApple]

Thanks sig !! : D .

Negative feedback is resistive only .

Output topology is transformer based , the two transformers inside the case are the output transformers , the 12AU7 preamp is anode capacitor coupled to the output pentode grid 1 .

Yeah its running AC on both heaters , there is hum but you have to put your ear next to the speaker to notice it , all cables are shielded and grounded , with twisted pair wire on the heater circuits .

 

[Sigurthr]

Very cool! Do you know the impedance characteristics of the output transformers? When I made my simple dual triode amp I tried using several transformers I had available but the only one that even approached usefulness was the original one from the 1920s that I salvaged along with the tubes. I then matched that to a speaker with a modern and common transformer, but the mismatch was still relatively strong.

I'm surprised you didn't opt for DC heating given that you made your own power supply section. I suppose any bias voltage formation is much more difficult with DC though, where as with AC it's relatively simple.

 

[DashApple]

Thanks ! : P ,

The transformers are designed for valve output use and are multi taped with impedances given for a 4 ohm and 8 ohm loads.

The circuit I used needed a Z of 2.5Kohms , the closet tape the transformers had was 2.6Kohms .

I was planning on DC , it would be easy to do on the 6.3V preamp tube , but I needed to order a HV regulator for the 50V needed on the 50EH5 tubes , just never got round to it .

The DC supply I built is just 2 fixed voltage regulators at 12 and 5V supplied from a 1.6Va transformer .

 

[ped]

I was going to ask why the toroid's where on the outside , but you have the output transformers inside so fair enough.

I always wondered why manufacturers never used DC for the filaments, to cut down hum....guess it might have been a cost saving thing.

Excellent work Ion.

 

[Sigurthr]

Usually it comes down to bias complexity and cost.

For example, the early Triodes I worked with had to have the filament/cathode held at a differential voltage of about 26V from the Grid (I can't remember if the grid was + or - 26v from the cathode) for proper operation. So, the filament power transformer was wound as an autotransformer section of the main power transformer. This resulted in a B+ line at 400v, a 0v leg, a -19.7v leg, and a -26V leg. If you use the -19.7v and 26V legs for filament power you get 6.3Vac that is 26V offset from earth/neutral. Then the grid is at neutral potential so that the antenna and LC tuning circuits aren't live.

The alternative would be to have the cathode at neutral/earth potential and then have a separate DC supply to bias the Grid.

If your tube doesn't need a large bias for the Grid then it is easy sailing, as I think the 12AU7 is. (555 would know for sure).